Wheel balancing is the process of balancing the weight of a tyre on the wheel so that it rotates evenly at high speeds. This involves adding small balancing weights inside the rim to counter any inconsistencies.
It prevents vibrations from passing into the car (often a cause of travel sickness in children) and prevents premature and uneven tyre wear. It should be carried out any time a tyre is mounted onto a wheel or if you feel a vibration in the car, usually in the steering wheel.