It's short for baby please.
Something you say when you really want someone to do something. Especially if it's someone you want to have sex with. Palringo uses this a lot.
friend1: I'm about to go to the store. You want anything?
friend2: Yeah, can you get me the big bag of funyuns?
friend1: I'll get you the small bag. The big bag costs too much.
friend2: But I want the big bag!
friend2: Bby pls ;-;