How ABS works...
ABS, or Anti-Lock Braking system, prevents the car skidding and retaining driver control by stopping the wheels locking up under heavy braking. Sensors in each wheel detect wheel speed, when the control unit senses one (or more) wheel(s) being significantly slower than the rest (or completely stopped) the control unit releases the braking pressure on that wheel. Once the wheel starts spinning again, the brakes are released. This is a fast process happening ~10 times per second. Juddering can be felt through the pedal and a juddering noise can be heard, which is normal.
Faulty sensor(s) are usually identified by ABS activation under light braking when the wheels aren't locked up or on ice/wet painted lines.