BRB is an acronym for Be Right Back and may mean a couple different things, based on context.
1. The character is going somewhere by himself to do something in-game.
2. The user will be AFK for a short time.
This could also be used as an acronym for bathroom break. However, this definition is rather moot. Most people would have to leave their keyboard (go AFK) in order to answer nature's call, which would mean they would "be right back", which is the more standard definition that BRB takes. If BRB is to mean "time to take a general bathroom break", it is advisable to set a time limit by saying "BRB, everyone be back in 5 minutes", since it would be ineffective to leave it open to allow everyone to return at their own leisure.
There is also an in-game emote activated by /brb. This is very handy if you wish to inform a member of the opposite faction that you are AFK or that you will return soon after moving your character away to do something.