Plain English Meaning
It is a way for controllers and pilots to talk to each other by sending typed messages back and forth over a special aviation data radio, instead of speaking on the radio.
Full Definition
A digital text-message communication system between air traffic controllers and pilots, carried over the Aeronautical Telecommunication Network (ATN) using VHF Data Link Mode 2 (VDL Mode 2) as the radio link. It allows controllers to send instructions, clearances, and information to the flight deck as data messages rather than voice transmissions, with the pilot responding by selecting a reply on the data link display.