Oversteer and understeer are two scenarios when a vehicle responds to the steering input of the driver. Both conditions refer to situations when the wheels lose traction and slide.
Oversteer is when the rear wheels lose traction causing the car to spin out of control. This happens during a turn when the driver applies more power than the wheels can handle.
Understeer is when the front wheels of a vehicle lose traction and slide outwards. Understeer happens when a driver is going too fast while taking a corner. In an F1 car, understeer happens due to braking too late or entering a corner too fast.
However, oversteering in F1 cars happens because of aerodynamic imbalance, tire pressure, etc. Teams try to balance between oversteer and understeer by adjusting the vehicle suspension settings.