Plain English Meaning
It is a way for pilots and air traffic controllers to send each other typed messages instead of talking by voice, using long-range high-frequency radio. It is mainly used over oceans and remote areas where normal radio does not reach.
Full Definition
A specific oceanic and remote-airspace data link communication system that allows controllers and pilots to exchange text-based ATC messages over long distances. CPDLC is the messaging service, FANS 1/A is the agreed message standard used by Boeing (FANS 1) and Airbus (FANS A) , and HFDL is the high-frequency radio data link that carries those messages where or coverage is unavailable.