Most Helpful Girl
Most Helpful Guy
Break + up = Breakup
So a break is either short for a breakup, or an almost breakup.
It means they are seriously debating if they want to be in a relationship with you and it could go either way. It means they aren't sure of it's worth it. It means giving a little distance, so the emotions (both good and bad) don't overwhelm them so they can really decide if they want to continue or not.
From what I've seen, it means a breakup more often than not. Sometimes it works out.