The tongue expresses what is in the heart. If a persons heart is good you see it in their saying.
If the tongue is used correctly then it has its blessing but if used incorrectly it has its dangers. Allah (subhanahu wa Ta'ala) has given us this tool (the tongue) to communicate and it is a blessing form Allah (subhanahu wa Ta'ala) which must be used correctly.
The gratitude we should show for this great blessing should include, using it for the obedience to Allah (subhanahu wa Ta'ala), for reading the Quran (and good Islamic materials), for speaking good, such as, enjoining good and forbidding the evil and in teaching the ignorant, in Dhikr, in worship, in sincere advice to our brothers and sisters and in many other things which enable us to get closer to Allah (subhanahu wa Ta'ala). It is true that ONE saying may enter a person into the fire and one saying may save a person from the fire and enter one into paradise.
The prohibitions of the tongue are that you do not lie, abuse, use foul language, backbite, and do not slander.
The tongue is protected by the teeth and then the lips which should act as a prison when indulging in loose talk. Allah (subhanahu wa Ta'ala) says in the Qur'an:
Successful indeed are the believers. Those who offer their prayers with all solemnity and full submissiveness. And those who turn away from Al-Laghw (dirty, false, evil vain talk, falsehood, and all that Allah has forbidden)
Here Allah (subhanahu wa Ta'ala) mentions the words "turn away from" not "leave" which shows that, refraining from vain talk is a characteristic of the believers.
Imam Shafi said, "If you wish to speak then it is upon you to think before you speak. If you think there is good in it then you should speak and if not then do not speak."
One searching for salvation against vain talk should ask themselves:
Will this saying of mine please Allah (Ta'ala)?
Will this saying of mine bring me closer to Allah (Ta'ala)?
Does this saying earn with it obedience to Allah (Ta'ala)?
If so then speak otherwise on should keep silent.
Sahl ibn Sa’ad (radiAllahu anhu) reported that the Messenger of Allah (salAllahu alayhi wasalam) said:
"Whomever guards what is between his jaws and legs, I will guarantee him paradise."
[Sahih al-Bukhari and Muslim]
This is one of the open paths to paradise. It is calling the people to come to paradise with the guarding of their tongues.