Research the history of AR. Although it started in the 1950s, the field of AR wasn't formally defined until 1997. Its development took decades, but by 2008 AR was being used in a variety of fields and the technology for future applications was growing by leaps and bounds.
Know how AR is being used currently. Turn on any televised football game and notice the yellow "first down" lines on the screen. Televised hockey games use AR to impose a red line during play of the game to help you follow where the puck is on the ice. There are a variety of iPhone applications that use AR and provide easier assistance for finding restaurants, measuring short distances and identifying constellations.
Know some of the future possibilities of AR or AR projects in production. The range of learning tools and devices to make our lives easier and more convenient are endless, but projects like recreating field trips for sick students who missed them are on the way. In years to come we might hold virtual conferences similar to those on the holodeck of the spaceship Enterprise. We could also buy a virtual window that projects the view from outside, using a camera on the exterior of the building, on to a solid wall.
Read a book that describes AR in detail to get the full scope of its potential. One such book is "Spatial Augmented Reality: Merging Real and Virtual Worlds," Oliver Bimber, Ramesh Raskar, 2005.