Anti-lock brakes (sometimes reffered to as ABS) are a computer-controlled component on a vehicle's braking system. It is designed to detect when wheels are about to lock up and reduce the braking power slightly to prevent a lock-up from occurring. Sometimes the ABS is part of the same computer system as the traction control system.
Anti-lock brake systems typically know when a wheel is locked up by reading the wheel speed sensor. When a locked-up wheel is detected, the ABS pump can lower pressure on that wheel (or axle on more simple systems) to let the brakes go a little, usually resulting in the wheel rotating again.