Plain English Meaning
A ground station that gathers GPS data from many monitoring sites, works out how far off the GPS signals are, and sends correction information up to a satellite so aircraft GPS receivers can fly more accurate approaches.
Full Definition
A Wide-Area Master Station is a ground facility within the Wide Area Augmentation System (WAAS) that collects GPS measurement data from a network of reference stations, calculates corrections for satellite and ionospheric errors, and uplinks those corrections to geostationary satellites for broadcast to WAAS-equipped aircraft.