A program that doesn't rely on binaries to be built, i.e can be built from source all the way down, versus requiring a compiler binary somewhere along the way. Look at Core's guix builds

I don't know guix but if it's source all the way down, what builds the source. At one point you'd be poking machine instructions into memory I guess if that's what you call "source".

Well, the point is that it's a small enough compiler that it can be audited and known to not have been tampered with. Look up GNU Mes and bootstrappable.org