It is (as usual) documented in the Handbook, here https://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/makeworld.html
But perhaps not written clearly enough for everyone (not intending to put OP in a group here).
RELEASE is a release version, intended for "normal" users (whatever they be).
STABLE is a development version. Developers are helped if users test / run / use this version and report any bugs they find. It is generally stable enough for daily use, and a lot of users runs this version.
CURRENT is the newest development version, where all the "bleeding edge" stuff is being developed. It might work one day and break the next, anyone trying it are advised to keep frequent backups of valuable data. The time you have lost dealing with any breakage can not be recovered.