The ABS modulator acts on commands from the brake computer. It uses rapid-acting solenoid valves to pulse brake fluid pressure. This keeps the tires right at the limit of traction. The driver feels this action as a vibration under their foot during hard stops.
Road grime or old fluid can cause the modulator valves to stick. If a valve sticks open, that wheel will have no brakes. If it sticks closed, the brake will drag or lock up, causing the vehicle to pull to one side.