Metafont was written by Knuth as a companion to TeX; whereas TeX defines the layout of glyphs on a page, Metafont defines the shapes of the glyphs and the relations between them. Metafont details the sizes of glyphs, for TeX's benefit, and details the rasters used to represent the glyphs, for the benefit of programs that will produce printed output as post processes after a run of TeX.
Metafont's language for defining fonts permits the expression of several classes of things: first (of course), the simple geometry of the glyphs; second, the properties of the print engine for which the output is intended; and third, 'meta'-information which can distinguish different design sizes of the same font, or the difference between two fonts that belong to the same (or related) families.
Knuth (and others) have designed a fair range of fonts using Metafont, but font design using Metafont is much more of a minority skill than is TeX macro-writing. The complete TeX-user nevertheless needs to be aware of Metafont, and to be able to run Metafont to generate personal copies of new fonts.