Check out the comparison between a new bicycle brake pad (right), and one that was used for several months (left). Those are V-brake pads and they work by pressing onto the metal rim of the wheel when the brake handle is pressed. Over time, this causes the brake pad to wear down. Here you can see an example of this having happened. Always check the wear of your brakes – if the grooves (look at the new pad on the right) are shorn, it means you’re in the risk of an accident.