In a nutshell, it measures a certain angle of the wheel of a car. There is the caster and camber of a tire. Camber is how a tire sits-if it lines up verticially, or has a tilt to it which could mean a serious problem. Caster is if the tire's suspension is off, the way it is supposed to hang vs a tilt that might face a slightly off angle. In the case of a caster being off, it is usually only within a few degrees, which isn't so terrible, but it isn't good. Camber tilt is usually the result of a loss of lugnuts and indicating that the tire is about to come off, or that there is some seriosu damage to the suspension. In either case for both Caster and Camber, problem with suspension can be serious, but the tools for actually checking your caster/camber can be done with a series of gauges, such as that the question is about. It can save you quite a bit to do it yourself, detect a problem before it gets out of hand or otherwise. on average, taking your car in to have these tests done can run from $70 to $100.