Brake calipers are a vital part of any disc braking system. They apply pressure to the brake pads which, in turn, press against the spinning discs and thus create the friction which slows the car down.
Depending upon the size and make of your car the calliper used will have differing numbers of pistons, but the principle is the same and so are the potential problems. After prolonged use, the calipers are subject to rust and corrosion, caused by either excessive braking or simply atmospheric conditions. Another factor which can lead to calipers malfunctioning is the heat created by on-off braking, something which can lessen the effectiveness of many other parts. If your brakes are worn or damaged then they may seize up, meaning you risk both a failed MOT and serious danger.