Plain English Meaning
A radar setup where the ground station asks a question and the aircraft's transponder answers back. The answer tells the controller which aircraft it is and how high it is flying, making the radar picture much easier to read.
Full Definition
A ground-and-air radar system in which a ground interrogator transmits coded signals that trigger a reply from an aircraft transponder. The reply provides controllers with a stronger, clearer radar return than primary (skin-paint) radar, along with the aircraft's assigned identification code and, when Mode C is active, its pressure altitude.
Context Anchor
Pilots encounter ATCRBS when using a transponder, setting an assigned code from ATC, selecting -reporting , or reading about and requirements.