You rely on a compiler to produce an executable, right? Well there are many "phases" that the code goes through before becoming machine code. "Compile time meta

You rely on a compiler to produce an executable, right? Well there are many "phases" that the code goes through before becoming machine code. "Compile time metaprogramming" is having control over one of those phases available from the language itself. Exactly how it is implemented is, well, implementation and language specific. But the fact that you can do it is quite useful. You can not only write programs, you can also write programs that transform or generate programs. The full extent of what you're allowed to do is implementation-dependent. In Lisps, for example, that phase is called the macro-expansion phase (of which there can be layers of) and generally acts as a source-to-source transformation from s-exp to s-exp (possibly with side-effects). Here's an example in a Gambit Scheme REPL: > when *** ERROR IN (stdin)@8.1 -- Macro name can't be used as a variable: when > (when #t #f) #f > (pp (lambda () (when #t #f))) (lambda () (if #t #f)) So you can see the 'when' macro expands to an 'if', and that has to happen _before_ evaluation. So the REPL is in fact a REEPL (Read Expand Eval Print Loop). Very useful.