An outline font editor that lets you create your own postscript, truetype, opentype, cid-keyed, multi-master, cff, svg and bitmap (bdf, FON, NFNT) fonts, or edit existing ones. Also lets you convert one format to another.
The most obvious aspect is that it is a drawing program like FreeHand, Inkscape or Illustrator which lets you draw the outlines of your letters. Unlike other drawing programs it expects you to draw many pictures at once (one or more for each letter) and collects them into a database.
It allows you to describe the way these pictures interact with each other (if you put one picture after the other then they should normally be separated by a certain distance -- the font's metrics, or if these two pictures are placed adjacent to one another then they turn into a third -- the font's ligatures, and so on).
Finally as a font editor it will bundle up all the pictures you have drawn, and all the metadata about how those pictures fit together, and will turn that bundle of stuff into a font that your computer can use to display text.