- It's a pass thrown behind the line of scrimmage to a receiver or running back who's already in an extended formation. The term "bubble" refers to the pass catcher bubbling back away from the line of scrimmage to give the quarterback a better angle to throw to him.
The goal is to have the receiver catch the ball in front of him, with his shoulders facing forward. At the same time, the receivers outside of him are blocking down the field.
The timing is crucial, because those outside receivers cannot start blocking until the ball has been caught. Otherwise, it's offensive pass interference. (Typically, officials won't call that as long as the ball has left the quarterback's hand.)