Plain English Meaning
It is a special radar that lets the tower see where every aircraft and vehicle is on the ground, even when they cannot see the airport through the tower windows.
Full Definition
A ground-based radar system used by air traffic controllers to detect and track aircraft and vehicles on the airport surface, including taxiways and runways. It provides a continuously updated visual display of surface movement, particularly useful in low visibility conditions such as night, fog, rain, or when controllers cannot see the movement directly from the tower.