Plain English Meaning
A radar on the ground that watches an aircraft on final approach and lets the controller talk the pilot down to the runway, telling them when to turn left or right and when they are above or below the correct descent path.
Full Definition
A ground-based radar system used by air traffic controllers to guide an aircraft along the final approach to a runway, providing the controller with continuous, highly accurate information about the aircraft's position relative to the desired glidepath (vertical) and course (lateral), as well as range from touchdown. The controller relays steering and descent instructions to the pilot by so the aircraft the correct to the runway.