Sometimes, deep scores get worn into brake rotors. This can happen if a worn-out brake pad is left on the car for too long. Brake rotors can also warp, that is, lose their flatness. If this happens, the brakes may shudder or vibrate when you stop. Both of these problems can sometimes be fixed by refinishing (also called turning or machining) the rotors. Some material is removed from both sides of the rotors to restore the flat, smooth surface.
All brake rotors have a specification for the minimum allowable thickness before they need to be replaced. This spec can be found in the shop manual for each vehicle.