BMW 325i What is wrong when you pass by a wet surface and you hit the break and the pedal starts to vibrate?

Normal operating of ABS Braking. Antilock Braking System. The brakes will release just as the wheel locks and apply again and release as the wheel locks. This could happen hundreds of times a minute or until you are off the slippery surface. makes for better steering on Ice and other slippery surfaces because it prevents wheel lockups