An ABS light comes on when the car computer finds a fault in the anti-lock brake system, causing the computer to deactivate that system. The car can still brake, but the wheels may lock up, causing the car to either spin or skid forward when braking.
A car's ABS activates when the car moves forward even though one or more wheels aren't spinning. The system rapidly releases brake fluid pressure from the locked wheel, allowing it to spin and keeping the car under control until it stops or the driver takes pressure off the brake. The system isn't used during normal braking situations, only those where the car lacks traction, such as a driver heavily applying the brakes or during drives on slick roads.
If the ABS light comes on, a driver should turn the car off then back on because it may have come on due to a computer system glitch. If that doesn't work, he can take the car to an auto repair shop. The system has a trouble code, which a mechanic can read to determine the issue. Possible causes could be the sensors or wires related to the system. In other cases, the ABS controller may need to be replaced.