Short for Subversion, a version control system. Basically, it tracks changes they commit to it, so they can go back and see what it looked like in past revisions.
It's like an incremental backup in a way, or perhaps a time-machine to the past of the mod.
Something like that anyway. I suck at explaining SVN and the like.
(Personally I prefer Mercurial aka HG, but that's just me.)