Oversteer is a condition where the front of the vehicle moves in a left or right direction more rapidly than the back can withstand, so the rear tires lose grip before the front tires do. This trait is common in high-powered rear-wheel drive vehicles, and in any situation where the front wheels have more grip than the rear wheels.
A high-performance vehicle with a managable amount of oversteer is desired in competitive drifting. A vehicle with an oversteer condition usually wears out the rear tires faster than the front tires.
There is a huge assortment of ways to fix oversteer in a vehicle, mostly through suspension adjustments, but adding weight to the rear or installing tires with more grip in the back are also options.
In a rear-wheel drive car, you can compensate for oversteer by reducing the amount of throttle use while turning. In general, you can avoid oversteer by reducing your cornering speed or how much you are turning the steering wheel at a given speed.