Literally "what you on". Can be used two ways. Either meant to see what you're doing, or used to see what you're trying to do (in terms of what your intentions are).
1: Wassup? Wyo tn?
2: Shit. Chillin prolly. Wat up?
1: Ima pull up in a few.
M: You're so damn fine!!
W: Ah thanks, but... wyo?
M: I'm on ulbs, wyo?
W: I'm only on u. Let's link.