What causes car to let off a screching sound when brakes are being mashed?

Screeching can be caused by two things and both indicate the brakes are worn to the point of needing replacement. The brake pads have rivets in them to hold them together and when they wear down to the rivets the rivets screech. The other thing is on disc brakes there is a wear sensor that drakes against the rotor if the brake pad get too worn.