Plain English Meaning
An equipment setup at the airport that listens to GPS satellites, works out the small errors in the satellite signals, and radios corrections to nearby aircraft so their GPS becomes accurate enough for a precision landing approach.
Full Definition
A ground-based GPS augmentation system that improves the accuracy, integrity, and availability of GPS signals in the immediate vicinity of an airport, primarily to support precision approach and landing operations. A LAAS ground station at the airport monitors GPS signals, calculates correction data, and broadcasts that data via VHF to aircraft within roughly 20–30 , allowing them to fly GPS-based to that airport. LAAS has since been redesignated as the (GBAS).