Brake pads, also known as brake shoes, are probably the most frequently replaced component of a disc brake system. Pads are steel plates with layers of high-friction material on their surface. Two pads are placed in the brake caliber, with the high friction surfaces facing inward, towards the rotor. When the brake pedal is depressed, hydraulic pressure is transferred through the braking system to the caliper, which clamps the pads together, on the rotor, to stop the car. This process produces a lot of heat, which results in the erosion of the friction material from the brake pads. When they are sufficiently worn, they no longer have enough friction to reliably stop the car and should be replaced. While squealing or squeaking noises while braking do not always indicate that you need to change the pads, they do indicate you should check your brakes.