Plain English Meaning
A radar at or near the airport that shows controllers where nearby aircraft are, so they can line them up safely for landing and takeoff. It tells the controller direction and distance, but not how high the aircraft is.
Full Definition
A short-range radar system used by air traffic control to detect and display the position of aircraft within roughly 60 nautical miles of an airport. It provides azimuth and range information but not altitude, and is the primary radar used for sequencing and separating arriving and departing traffic in the terminal area.