To back-taxi is an aviation term meaning to taxi on an active runway against the direction of takeoffs and landings. Most folks who have flown are familiar with the idea of a taxiway - a paved path for airplanes to move along on the ground. These are almost always used to get airplanes to and from runways; most large airports and many smaller ones have taxiways which can be used to reach either end of a runway (as runways can be used in either direction, for takeoffs and for landings). However, some smaller airports or smaller runways may not have a taxiway at all, or may not have one which reaches the end. In that case, it is sometimes necessary to taxi along the runway opposite that of the traffic using the runway for takeoff or landing, in order to reach the end of the runway, turn around, and take off. This is called back-taxiing, and since it means your aircraft is blocking the runway for some time while you're doing it, you should (unless you're under ground control at the time) always announce that you are back-taxiing on the CTAF or even tower frequency.