This is used a lot in corners because it controls the wheel spin when under and over steering. They mean simply to which end of the car runs out of grip first as the tyres only have limited amount of grip. In an under steer, the front axle and wheels runs out of grip first and in Over steer the rear end runs out first. When this happens it is like a handbrake skid. Traction Control is the device that tries to keep the car stable by adjusting the wheel movement in these turns.