Stands for "be right back". Used in IMing. Some people actually do come right back after saying brb. Most people don't come back after saying brb. Brb is used for an excuse (some of the time) so that the other will stop talking to them, but they won't have to go offline. Usually responded to by "kk" which means "kay, kool"
Guy: HAHAHHAAHAHAHAHAAHHAAHHAHAHAHAHAHAHH THAT WAS SO HILARIOUS!!!!!
Girl: Um...that wasn't funny..at all.
Guy: HAHAHAHAHA...aha...aha.... SO yeah WHATS up? HAh, isn't that so FUNNY how IM captilizing SO randomly??
Girl (thinking that this guy has spazz issues): brb.
24 hours later...
Guy: you there...??