Typically this results in smaller class sizes and faster startup times. A similar variable, grep-regexp-alist, tells Emacs how to parse output from a grep command see Grep Searching. By convention, bytecode file names end in.
This is known as compilation. The :init clause names the constructor function. Elide meta Var meta docstrings, file and line info, etc will be compiled into strings in the constant pool of the compiled classes. This indirection via the var is one of the ways that Clojure provides a dynamic runtime environment.