It means either:
A: associating it with basketball .. you yell "ballin" wen sum one makes a basket.
B:You popped(took) an E pill .. Down here in Jersey we call them ballers and when you begin 2 feel it(begin 2 ball) then your "BALLIN"
C: You madd rich and got money .. that you can flaunt and spend like it aint shyt..so you "ballin"
A: I shot a paper ball in the trash and it made it so my boy yells "BALLIN" in accordance to the fact that i made it.
B:Yo I popped my baller a little bit ago and now im "ballin" hard as hell.
C:I got money to the ceilin bro..Im "BALLIN"