Coming from the movie Step Up 3D BFABB is an acronym meaning Born From A Boombox. "Some people learn to dance, others a born to."-Step Up 3D. It basically means that you're so good at dancing that you we're given that talent to do so when you were born. The phrase sort of means it's almost like a boombox gave birth to you and you inherited the genes from the boombox that creates music that creates DANCING.
*Luke sees Moose's sick dancing*
Moose:"What's a BFABB?"
Luke:"Born From A boombox."