Brakes use friction to convert the kinetic energy of your moving vehicle into heat. If the brakes get wet, the brake shoes or brake pads may produce less friction. The brakes will grab, apply unevenly, or become weak. When you try to brake, your braking distance may increase, your vehicle may pull to one side, or the wheels may lock. On a wet road, locked wheels can cause a skid (or even a jackknife if you're pulling a trailer).