RELEASE is the release, -STABLE is the minor branch for bugfixes and minor new features that don't break backwards-compatibility ("stable ABI"). -CURRENT is, well, current - it's the tree the developers are working on, and is the current state of FreeBSD, where releases are archives at a particular point (more-or-less).

I suspect much of this goes back to when development wasn't quite so public, so these are names that are meaningful to the developers, not the public.