What are Multiple Master Fonts?
Multiple master fonts, created by Adobe, are customizable PostScript fonts. The multiple master font format describes a typeface's characteristics using linear design axes, which include weight, width, optical size, and style.
Using a multiple master font as a source, you can use a program such as Fontographer to create many different weights, sizes, and styles for the font. If the particular master font you choose has more than one "axes", especially the "optical size" axis, then you can create versions of the font that are optically adjusted for their specific sizes. So, a font designed for 10 point type will be more spaced out with thicker serifs and a larger x-height, while a font designed for 72 point will be more tightly spaced, with thinner serifs.
It should be noted, however, that multiple master fonts are not used that frequently in the print production industry. In fact, I personally don't know anyone who's ever used them.
Reference: Adobe
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