Okay, if you've heard someone say "back that thing up," it's an idiom that means "Go back a couple steps in the conversation. Something you've just said doesn't make sense or something you've just said doesn't sound like it's true/accurate." You may also hear it as "Back up the truck a minute, there, buddy," or something similar. It's very informal.
Can you help me with this?, there's a t-shirt where you can see an RV and you can read "Back that thing up". It's supposed to be funny, but I don't get it. Can you explain it? Here's an image: http://s7d2.scene7.com/is/image/JCPenney/DP0703