Plain English Meaning
A radar at the airport that lets controllers see where nearby aircraft are and how far away they are, so they can guide them safely in and out.
Full Definition
A short-range radar system installed at or near an airport, used by air traffic control to detect and display the position of aircraft within roughly 60 nautical miles of the airport. It provides azimuth (direction) and range (distance) information, but does not directly measure altitude. ASR is used to sequence and separate arriving and departing traffic in the terminal area and can be used to provide ASR approaches when needed.