You know how the Earth turns and stuff? And how the stars appear to move as a result? This gets to being a pain in the ass if you try to do any long-exposure shots of the night sky: you can normally only get about a 30 second exposure, tops, before you start to see star trails in your photo. That can sometimes be a good thing, but if you want to get those really intense shots of distant nebulae and the like, your shutter will have to be open a lot longer than that.
A Barn Door Tracker is a contraption that lets you mount the camera and adjust the thing to follow the sky. These can range from simple wooden ones you wind up yourself to follow the sky up to complex custom motorized devices.