Plain English Meaning
The equipment installed at an airport that listens to GPS satellites, works out how much the GPS signals are off, and broadcasts a correction to arriving aircraft so they can fly very accurate approaches to the runway.
Full Definition
The airport-based equipment portion of a Ground-Based Augmentation System (GBAS), consisting of multiple precisely surveyed GPS reference receivers, a central processing unit, and a VHF data broadcast transmitter. The facility receives GPS signals, calculates corrections and integrity data based on the known surveyed positions, and broadcasts that information to aircraft on so they can fly to at that airport.