A bracer (or arm-guard) is a strap or sheath, commonly made of leather, stone, or plastic, that covers the ventral (inside) surface of an archer's bow-holding arm. It protects the archer's forearm against injury by accidental whipping from the bowstring or the fletching of the arrow while shooting, and also prevents the loose sleeve from catching the bowstring. They normally only cover part of the forearm, but full-length bracers extending to the upper arm are also available, and other areas have been covered by some archers. In addition, chest guards are sometimes worn, usually by female archers, to protect the breast. With some combinations of non-baggy clothing and bows with a larger distance between the bow and the string, the archer may not need to wear any bracer.

Source: Wikipedia