Wheel balancing, also known as tire balancing, is the process of equalizing the weight of the combined tire and wheel assembly so that it spins smoothly at high speed. Balancing involves putting the wheel/tire assembly on a balancer, which centres the wheel and spins it to determine where the weights should go.
In essence, wheels and tires are never exactly the same weight all around. The wheel’s valve stem hole will usually subtract a small amount of weight from that side of the wheel. Tires will also have slight weight imbalances, whether from a joining point of the cap plies or a slight deviation from perfectly round, because that kind of perfection is impossible to achieve. At high speeds, a tiny imbalance in weight can easily become a large imbalance in centrifugal force, causing the wheel/tire assembly to spin with a kind of “galumphing” motion. This usually translates into a vibration in the car as well as some very irregular and damaging wear on the tires.