WAAS is an acronym for Wide Area Augmentation System. WAAS provides correction data that allows specially equipped GPS receivers to calculate their position more accurately.
Correction data is created from measurements taken by several ground-based stations that compare their precisely known location with locations calculated from GPS signals. If any differences are found, they can be used to create correction data that's broadcast from WAAS satellites.
WASS can help correct for errors caused by clock and orbital variations associated with individual GPS satellites as well as distortion of GPS signals as they pass through the ionosphere.