Plain English Meaning
A way for the cockpit and air traffic control to send each other typed messages over the same VHF radio band normally used for voice. It is one of the underlying 'pipes' that carries digital ATC messages.
Full Definition
A digital radio communication system that uses VHF frequencies to send data messages between aircraft and ground stations. VDL-2 is the primary data link used to carry Controller-Pilot Data Link Communications (CPDLC) in domestic airspace, allowing controllers and pilots to exchange text-based and instructions instead of, or alongside, voice radio.