Some brake pads have shims built into them in order to deliberately make a grinding noise when they wear out, or sometimes a squeaking noise.
This is then designed to alert the driver to the problem so that they get their brake pads replaced. If the brake pads continue to grind then the calipers might connect directly with the brake rotors and this can cause them to cut out a groove in the rotor meaning that the rotor needs to be either replaced or turned around. You might also need to readjust the calipers or add a lubricant.
While there are other brake parts it is the brake pads and brake rotors that control most of their function. Ensure at all times that your brake pads and brake rotors are in good condition. If you think that your brake pads are wearing down, or that your brake rotors might be warped then you should look into getting those brake parts repaired. Look out for your brakes becoming less responsive, for them becoming uneven, or for vibrations being felt in the wheel.