Plain English Meaning
A landing system that takes regular GPS signals, fixes the small errors in them using a ground station at the airport, and sends those corrections to your aircraft so you can fly an accurate approach all the way down to the runway.
Full Definition
A precision approach and landing system that uses GPS signals corrected and enhanced by a ground station at the airport. The ground station measures errors in the GPS signals it receives and broadcasts correction data to aircraft on a VHF datalink, allowing them to fly precise approaches comparable to a Category I, II, or III without needing the traditional ground-based and transmitters.