An Anti-lock Braking System (ABS) prevents the wheels of a car from locking up when the driver applies heavy pressure to the brake pedal, usually during an emergency braking situation.
Without ABS, the front wheels of the car would likely lock up and start skidding across the road surface under heavy braking.
As soon as the front wheels are sliding they will not respond to steering input making it impossible for the driver to try and steer clear of a hazard ahead.
ABS works by constantly monitoring the rotational speed of each wheel. If it senses that any wheel is rotating at a slower speed than the speed of the car (which will occur just before the wheel is about to lock up) then it will automatically release braking pressure on that wheel to prevent it from locking up.
As soon as it senses that the wheel is rotating faster than the speed of the vehicle then it will reapply brake pressure, and then the process repeats.
Most ABS systems can complete this process multiple times per second making it almost impossible for the wheels to lock up.
This enables the driver to maintain steering control of the car to assist in avoiding a hazard ahead.
Practically every new car on sale today includes an ABS system as a standard feature.