Plain English Meaning
A special code you dial into your transponder to tell controllers, without using the radio, that your radio has failed.
Full Definition
The four-digit transponder code (7600) that a pilot sets on the aircraft transponder to alert air traffic control to a two-way radio communications failure. When ATC sees 7600 on radar, they know the pilot can no longer communicate by radio and will adjust handling, clear other traffic, and provide assistance based on lost-communication procedures.
Context Anchor
Used during communication , especially when flying under air traffic control and radio is lost.