The ABS module is the electronic brain of your anti-lock brakes. It processes signals from the wheel speed sensors. If it detects a wheel slowing down too fast, it controls the hydraulic pump and valves to prevent a skid. It also runs self-checks every time you start the car.
Heat and vibration under the hood can cause the module's internal solder joints to crack. When this happens, you might get random warning lights or communication errors. Rebuilders can often repair these modules by resoldering the connectors.