Plain English Meaning
It's a way for pilots and air traffic controllers to send each other typed messages instead of talking on the radio. The messages travel through a satellite (MTSAT) using a standard format called FANS 1/A. Aircraft equipped with this can be cleared on routes where voice radio coverage is limited.
Full Definition
A specific data-link communication capability that allows controllers and pilots to exchange text-based messages (clearances, requests, reports) through the FANS 1/A protocol, transmitted over satellite communications using the Japanese MTSAT satellite system. It is one of several authorized communication methods listed for aircraft operating on certain RNAV departures and oceanic routes where voice is impractical.