The idea is very simple. A master is an outline of a master design, which is most often designed by a human. The computer can interpolate two or more master designs to obtain arbitrary intermediate outlines called instances. Thus we have not only a single glyph for every character but an infinite number[C-16] of instances, described by their coefficients of interpolation from some number of master designs.
[C-16] In computer science, the term "infinity" means "a very large number".
We can have up to 16 master designs in a Multiple Master font, and they can be organized around four axes of interpolation. Each axis of interpolation is defined by two master designs placed at extreme locations. We can also decide that certain master designs will appear not at the extremities of an axis but at "intermediate positions" along a given axis.
Here we have a practical example: the font Adobe Kepler MM is a Multiple Master font with three axes, respectively called "weight", "width", and "size". In the left part of Figure C-14, we see the three axes of the font represented in Cartesian form. The encircled glyphs take on the extreme values and are therefore master designs.
Now let us look at the technical side. Multiple Master fonts have a well-deserved reputation for being quite complex. Here is some information that will help us to understand Adobe's strategy:
According to the nature of the axes that have been defined, some properties remain fixed and others vary. ...