Brake pads are commonly made out of organic, semi-metallic or ceramic friction materials. When you press the brake pedal, brake fluid is forced from the master cylinder to the calipers, in a disc brake system. This moves one or more pistons in the caliper; which forces the brake pad against the rotor. Over time, your brake pad material wears away.
It is important to change your brake pads before the fiction material is completely worn away. If brake pads are allowed to wear to the point there is no more friction material left, it can severely damage brake rotors, cause