When you brake in an emergency with a car that has ABS, you should brake as hard as possible and steer to avoid the obstacle or hazard. When the system is active, the brake pedal vibrates and pulses and make a certain sound. This is normal, and means that the system is working as it should.
ABS operates by automatically reducing the braking force on wheels that are about to lock. When there is no longer any risk of the wheel locking up, the system increases the braking force again. This process is repeated several times per second, which helps you maintain steering control of the car.