What happens if i don't change my brake pads?

Once the pads are worn off you will have the metal backing of the brake pad making contact with the rotor. It will, in just a few stops ruin the rotors meaning you will have to replace the rotors. If you continue to drive it this way it will eventually cut completely through the rotor and it will break causing the brakes to lock up on that wheel and you can have a serious accident.