This photo shows the buckle guard on an english saddle. The buckle guard can range from barely large enough to cover the girth buckles to large enough to cover the billet straps completely- as shown here. The buckle guard exists to prevent the buckle of a english girth from creating visible wear on the outside of the flap. Without the buckle guard, the friction of a rider's leg pressed against the girth buckle, can create wear marks on the piece of leather seperating them. But with a thick piece of leather serving as a girth buckle this wear is prevented and many times the saddle feels more comfortable to the rider.
The buckle guard is one place many saddle manufactuerers choose to mark their saddles. The saddle shows has the model name and the maker, and at the top is stamped with the tree size (a number ranging from 27-33, with 31 being average) and a serial number or other manufactuerer's number.
On billet guards securely anchored, not free moving on the billet straps, the billet guard and can should be oiled and conditioned as frequently as the seat and flaps of the saddle require it.