What does the phrase all but mean?

I need to know what the saying "all but" means. Like "someone is all but invisible" does that mean they are ALL but invisible or that they ARE invisible? HELP!

Neither. It means they are almost invisible; practically invisible.

By the say, I should not have said "they," I was following your lead. "Someone" is singular; a better answer is "...he is almost invisible..." Or if you want to be politically correct, "...he or she is almost invisible."