Why do brakes squeak?

if they screech for the most part it is a good indication you need new pads there is a metal indicator that when brake pads wear down will start scraping against the rotor now if you know the break pads are good or new screeching is common for the first few days if it continues it could be that you did not apply brake grease to the back of the pad where the pad meets the caliper if that's the case take them off and get a couple tubes of brakde grease (not that expensive) and put them on the back of the pads BUT DO NOT PUT THEM ON THE ACTUAL PAD WHERE IT WOULD CONTACT THE ROTOR that will make your brakes fail