Most modern vehicles are equipped with an Anti-Lock Braking System (ABS), a system that electronically pulses the brake pads under high braking force in order to avoid lockup, shorten braking distance and maximize vehicle control.
This system works by monitoring the speed of each individual wheel and comparing it to the speed of other wheels. It also cross-references this data with other sensor inputs such as vehicle and engine speed and uses all of this information to determine whether or not a vehicle is approaching brake lockup.
The sensor at each wheel is a magnetic device, meaning it tends to pick up metal debris and occasionally requires cleaning to continue effectively sending clear signals to the ABS computer. Less commonly, the sensors will be damaged or otherwise rendered useless and need to be replaced. Clean and functioning ABS sensors will ensure you have maximum braking potential when the time comes to jump on the brake pedal.