Mere bhai....ABS is abbreviation of anti-lock breaking system which means that the system dont let the tyres jam in case of hard braking because once the wheels are jammed during breaking....u cant steer the car left or right and the car keeps on moving in the same direction where it was before breaking. So here comes ABS, it keeps ur wheels rotating so that u can steer left or.right to avoid collision but on the other hand it keeps on slowing ur speed by continuously knocking on wheel discs. Thats why u hear knock sound when ABS engages.
Remember ABS only engages when tyres slip or jams due to breaking.....it might start working on slippery roads even if u break with a light foot.
Hope u understand this technology and its use.