Plain English Meaning
It is a way for pilots to speak to controllers over the ocean by routing the radio call through a satellite instead of a ground station, used in parts of the world where ground-based radio does not reach.
Full Definition
A satellite-based voice communication service that lets pilots and air traffic controllers talk to each other by radio over long oceanic and remote areas where normal VHF radio cannot reach. The service was carried by the Japanese Multifunctional Transport Satellite (MTSAT) system, which provided both aviation communications and weather observation coverage over the Asia-Pacific region.