There are primarily six prohibitions on Yom HaKippurim:
1) Creative activity
2) Eating and drinking (drinking is considered a form of eating)
3) Washing
4) Smearing oils on the body
5) Wearing shoes
6) Sexual relations (Shulchan Aruch, Orach Chaim, 611:1)
The source for these prohibitions is the Mishna in Yuma (73b), which says that eating and drinking, washing, smearing oils, wearing shoes and sexual relations are prohibited on Yom HaKippurim. Rashi explains that the Gemara derives these prohibitions, from the Mitzvah of "affliction" on Yom Kippur.
The Torah says, "…in the seventh month, on the tenth day, afflict your souls and do not perform any creative activity…(Vayikra 16:29)." We see that the Torah commands two things:
1) To afflict ourselves
2) Not to perform creative activity
Thus, Yom HaKippurim is the Shabbos of Shabbosos (Vayikra 16:31). Not only do we refrain from creative activity as on a regular Shabbos, but we also "afflict ourselves," which means that we refrain from these other five activities. The Gemara explains that the definition of "affliction" on Yom HaKippurim is abstaining from eating and drinking, washing, smearing oils, wearing shoes and having sexual relations. The main distinction, in terms of the Halachos of the day, between a regular Shabbos and Yom HaKippurim lies in these additional five restrictions. "Shabbos" means to rest, or to desist; therefore, it would seem that what makes Yom HaKippurim the Shabbos of all Shabbasos is the additional five afflictions - the additional abstentions. Yom HaKippurim is the Shabbos of all Shabassos, because we refrain from more activities than any other Shabbos.
Two questions emerge from all this:
1) Why is Shabbos defined by abstention?
2) How do the five afflictions enhance the concept of Shabbos?
Obviously, these two ideas are going to have infinite depth to them, but let us see what we can unearth.
The Gemara (Berachos 57b) says that Shabbos is one-sixtieth of Olam HaBa.
Why does the Gemara use the example of one-sixtieth, and not one-third, or some other number? Shimmy Steinmetz once explained to me that the Gemara gives specifically one-sixtieth as the example based on the Halachah of Batel BeShishim. For example: if some milk falls into a cholent, if the milk that fell in, is less than one-sixtieth of the total cholent, then the milk is considered as if it is not there. Why do we consider it to not be there? Since it cannot be tasted, it is as if it's not there. One-sixtieth is the limit of taste. Therefore, when the Gemara says that Shabbos is one-sixtieth of Olam HaBa, it means that Shabbos is a taste of the world to come.
This idea has many ramifications, and the more you think about it, the more you see its many facets. However, let’s focus on the concept of abstention. As you may know, Malachim only have one leg - which is why we stand with our feet together during the Shmonah Esrei and Kedushah. Why do I care that angels only have one leg? The idea is that they cannot walk. There is no such thing as progress in the next world. Today we do the work and tomorrow we receive the reward (Eruvin 22a). This world is the only time when we can create our personality. It is only here that we can perform the Mitzvos and progress on our quest to completion. After 120 years, whatever a person has built, he retains for eternity. However, what he didn't build during his life, those parts of himself that are still dilapidated, remain that way as well. The world is the time to build the house you will dwell in for eternity. Whatever beautiful furnishings you have acquired make your house an eternal delight. However, whatever leaks you ignored, will be dripping for a long time. The seat you get in the Bais Medresh up there depends on what you did with yourself down here. Once you get your seat, it becomes a Makom Kavuah.
This idea is reflected in Shabbos as well. We spend the whole week creating, and building. However, on Shabbos, everything comes to an abrupt stop. Creative activity is prohibited on Shabbos. Whatever you prepared for your Shabbos meal, you have. However, whatever you failed to set aside for Shabbos, is Muksah, and you cannot even move it. Thus, Shabbos is a taste of the world to come in that all creative activity ceases, and only what you previously prepared can you enjoy. Abstention is a fundamental definition of Shabbos, because it is supposed to provide us with a taste of the world to come.
Even with this understanding, we still have a problem: how do the five afflictions enhance this concept of Shabbos?
If we can understand some of the depth behind the one that seems to be out of place, then I think the idea will become clear in the others as well. We abstain from five things: eating and drinking, washing, smearing oils, wearing shoes and having sexual relations. Four of them intuitively make sense. We don't eat, wash, smear oil, or have sexual relations, because we are trying to distance ourselves from physicality. But, what in the world does not wearing shoes have to do with anything?
In his commentary to Pirkei Avos (1:1), Ruach Chaim, Rav Chaim of Voloshin discusses the singularity of Moshe Rabeinu. Among the many ideas that he brings up, is a passage from the Zohar (also in Shmos Rabah), that comments on the pause between "Avraham, Avraham (Ber. 22:11)". When G-d calls to Avraham, He says "Avraham" twice. Later, G-d also calls to Moshe, saying his name twice (Shmos 3:4). Why does G-d have to say their names twice? Once is not enough? They couldn't hear G-d, so He had to say it louder?! I don't think so.
The Zohar comments that there is a subtle difference between the way that G-d called to Avraham, and the way G-d called to Moshe.
When G-d says, "Avraham, Avraham," there is a pause between the two names (the Trope called a "P'sik" - a linguistic pause). However, when G-d calls to Moshe, there is no pause; G-d says, "Moshe Moshe (there is no "P'sik")."
Rav Chaim of Voloshin says that this hints at the tremendous level of Moshe Rabeinu.
Similarly, the Gemara (Chulin 89a) says what the Torah writes regarding Moshe (and Aharon), is greater then what it says regarding Avraham. By Avraham it says, "I am dust and ashes (Ber. 18:27)." However, by Moshe (and Aharon) it says, "What are we (Shmos 16:7)?"
What does all of this mean?
Rav Chaim explains that a name, in Lashon HaKodesh (The Hebrew words in the Bible), is more than a simple convention.
In any other language, a name is simply a word that people agree upon to refer to a specific object. Were it not for names, we would not be able to speak intelligently about anything. However, In Lashon HaKodesh, a name is more than a convenient way to refer to something; it is the thing itself. Just like the word Sheim (name), with one vowel change becomes Shom ("there"), so too the name of an object in Lashon HaKodesh is its "there." That is to say, that an object's name is its purpose; where it is going. Of course, as Rav Tatz always points out, the word for Heaven, Shomayim, can be pronounced as Shomim, which means "there(s)." Shomayim, is all of the there(s). Anything that can be said to be a "there," an endpoint, is called Shomayim. What is important for our discussion though, is the idea that a name is similar to a mathematical equation, in that it is an object's essence. Therefore, a person's Hebrew name, is his fundamental self. That is why when Adam HaRishon names all of the animals, it says, "And whatever he called it, the animal was its name." Quite literally, since Adam could perceive the objects essence, "the animal was its name." The letters organized in such a fashion, are what make the person what he is. G-d created the world through speech, and speech requires letters. The way that G-d organized the letters created that object.
This is how Rav Chaim of Voloshin explains the repetition of "Avraham, Avraham," and "Moshe Moshe." The first name (Avraham, Avraham)refers to the essential person, and that is where the soul truly exists, in the conceptual world of the letters. Only the smallest portion is manifest in the body… (Listen to what Rav Chaim says,) "Because the body is only like the shoe for the soul. Just as the shoe only clothes the very bottom extremity of the body, so too the body only clothes the very bottom portion of the soul. A shoe is called a body, in that a body is considered the end of the feet of the soul."
When Moshe sees the burning bush, he goes out of his way to find out why it is burning. Hashem then tells him to take off his shoes (Shmos3:5). Rav Chaim explains that Hashem told him to take off his body. You see, the second name (Abvraham, Avraham) refers to that portion that is expressed in the body. Every other person, even Avraham Avinu, had a separation between his essential self and that portion expressed in his body. As long as a person has an ego, an identity outside of his source, then a body separates his essential self from that portion expressed in the world. When Avraham is arguing with G-d to try and save Sodom and Amorah, he says, "But [who am I to be arguing?], I am dust and ashes." It is true that Avraham Avinu considered himself to be lowly, to be only dust and ashes. That is a tremendous level, which few others have ever reached. However, he still considered himself to be something! Moshe says, "What are we?" He considered himself to be "what." Moshe considered himself to be nothing; he had no identity outside of his source. Therefore, there is a "P'sik," a pause between "Avraham, Avraham," and none between "Moshe Moshe." This is why Moshe merited to give the Torah, and why anything Moshe said is considered a Torah law…but, that is a subject for another time.
What we see from this is what it means to take off one's shoes. The removal of the shoes means the removal of the body. It means to suppress the body so that we stand before G-d as angels, with no identity outside of Him.
Yom HaKippurim is the ultimate taste of the world to come; it is the Shabbos of all Shabosos. On Shabbos, we desist from creating, because we taste Olam HaBa. On Yom Kippur, we are almost in Olam HaBa. On Yom HaKippurim, many have the custom to wear a Kittle, or even white clothing, because it represents purity; it is the color of Malachim. We do not eat or drink, wash, smear oils, wear shoes or have sexual relations, because we are trying to limit our expression in the physical world. We are trying to take off our shoes. Far too often, we spend the year serving the body. On Yom Kippur, we rid ourselves of our bodies, and we go Halchically barefoot; we do not wear leather shoes.
I think that this is the reason why we remove our shoes in a place of holiness. It is not a Din in manners; there is no such thing as meaningless etiquette when it comes to our relationship with G-d. Rather, a place of Kedushah is a place of connection to Hashem, and that is no time for separations. Yom HaKippurim is the time when G-d is closest to us, even the evil inclination has no power on Yom HaKippurim (the numerical value of HaSatan is 364, because he only has power over us for 364 days of the year). We should all merit to remove our shoes, and identify with our source. Through this, we will merit a Chasimah Tovah and the final redemption, soon in our days.