The primary function of a brake pad is to slow or stop the motion of a machine or vehicle (car, truck, train, motorcycle, etc..). Brake pads are a component of disc brakes used in automobiles and other applications.  Brake pads convert the kinetic energy of the vehicle or machine into thermal energy by the use of friction.   A brake caliper typically has two brake pads inside of it with their friction surfaces facing the rotor.  When the brakes are hydraulically applied, the caliper squeezes the two pads together onto the spinning disc to slow or stop the vehicle or machine. The brake caliper is like a hand that squeezes the disc.   When a brake pad makes contact with a disc it transfers small amounts of friction material (transfer layer) to the disc surface turning it dull gray. The brake pad and disc then stick to each other, providing the friction that stops or slows the vehicle or machine.