Ballin' is a slang for someone hype.
It could either be one of these :
1) To play basketball.
2) One who is in a state of being extremely rich and likes to show off how rich he is.
3) Living the life.
1) That dude is ballin' at Rucker park !
2) He got bling & mad cash ! He ballin' !
3) He owns a Chrysler 300C & a Cadillac Escalade both on 24's ! He Ballin' !