m3makefiles are in fact quake programs. quake is an interpreter for a small, but powerful programming language. It's an imperative language with variables, arrays, tables, conditionals, loops, and procedures. The full features of that language are available if required, but very few m3makefiles need this extra generality.
Described with the builder are the facilities (many of them programmed in quake) that cm3 users should need. Most m3makefiles are simply sequences of calls to these predefined procedures.