It means you turn the front wheels, but the car does not turn as sharply as it should because the front wheels skid.
Oversteer is the opposite: you steer, and the car rotates farther than it should, because the back wheels skid.
Or to use a couple of racing definitions:
Understeer is when the front of the car hits the wall first; oversteer is when the back of the car hits the wall first.
Oversteer is when the passengers are scared; understeer is when the driver is scared.
