Parking sensors use ultrasonic or radio waves that reflect off of objects behind the vehicle to let the driver know how close they are to hitting the object by emitting a beep as a warning. They allow the driver to be more confident while parking and driving in reverse.
Returning radio and ultrasonic waves that bounce off of objects behind the vehicle are registered and read by the vehicle's computer to determine how large the object is and how far or near the vehicle is to it. Measurements are done by analyzing how long it takes for the waves to bounce off of the object. A majority of sensors and computers aren't calibrated to register any object that is more than five to fifteen meters away.
Some parking sensors are calibrated to detect moving objects. After the sensor registers a signal or wave that is received too quickly, all of the sensors located at the back of the vehicle send out a wave or signal at the same time. Whether or not the vehicle's computer is able to determine if the object is moving depends on the sensors that pick up the returning waves.
One benefit of parking sensors is that insurers may offer lower premiums to drivers with cars that contain these sensors.