Plain English Meaning
A small ground station at the airport that listens to GPS signals, works out how far off they are, and sends the correction to nearby aircraft so their GPS position is far more accurate for approaches and departures.
Full Definition
A satellite navigation enhancement system that uses a ground station at or near an airport to correct GPS signal errors and broadcast those corrections to aircraft via VHF data link, enabling highly accurate approach, landing, and departure guidance within the local .