GPS is an acronym for Global Positioning Satellite. Devices with GPS built-in are aware of their location on the surface of the earth. This information can be useful in augmented reality systems because it allows the AR system to know what the “real” reality of the device is, pull in correlating data (like nearby map features, weather, other GPS data encoded objects and so on). In conjunction with a compass, GPS can have a sense of what direction the device is facing–perhaps enough to place useful data or artwork into a live video overlay.
GPS-based AR applications require the system to know the location of the viewer, usually approximated as the location of the device used to run the AR application. This is in contrast to Glyph-based AR application, which require possession of a Glyph to display the data or augmentation.