What does the phrase if you're wating on you you're backing up mean?

The phrase "If you're waiting on me, you're backing up" is typically a response when one (Andrew) asks another (Ben) if they are ready to go.

Ben responds with "If you're waiting on me, you're backing up" because he is already going (or ready to go) and if Andrew waits for him (stands still) he is essentially backing up. The logic from this is related to distance between two objects. Andrew standing still (0) and Ben moving forward ten feet (+10) equates to the same distance as Ben standing still (0) and Andrew moving backwards ten feet (-10).

Conclusion: If someone says to you "If you're waiting on me, you're backing up" they are ready to go and want you to get moving also.