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!
friend1: NO.
friend2: Bby pls ;-;
friend1: Kfine.