Oversteer, or being loose, occurs when a car loses traction in the rear end while turning, possibly spinning out, and generally making it hard to make the car go where you want. It is the opposite of understeer.
See oversteer in action in this video - as the black and red truck exits the straight to the right it has a very hard time lining up in the next straight, causing it to scrub a lot of speed and eventually spin out:
If you're racing drift, oversteer is the name of the game! Still though, with drift, the key is controlled oversteer - the rear end should swing around, but not too much!