The brake pad sensor is a single-use, but nevertheless essential piece of equipment. It performs the job of alerting you when your brake pads are wearing thin, which in turn triggers a warning light to indicate that it's time to replace the brakes.
In constant use or when placed under high levels of friction, a car's brake pad will inevitably wear down. When the pad has worn to a pre-programmed point, the brake pad sensor makes contact with the metal of the brake disc and completes an electrical circuit; this action sends a signal to the dashboard in the form of a warning light.
Once a brake pad sensor has completed a circuit through contact with the rotor, it cannot be reused. Every time you change your brake pads you also need to install a new brake pad sensor. It's recommended that you replace the brake pad sensor even if it has not made contact.
The brake pad sensor is a real advance in automotive safety and your car will not pass its MOT if it has a faulty sensor.