Graham cracker crumbs are simply graham crackers that have been crushed or ground into fine, sandy crumbs. You can make them at home by pulsing crackers in a food processor or crushing them in a sealed bag, or buy them ready-made in a box.
Their main use is as the base for pie and cheesecake crusts: the crumbs are mixed with melted butter and usually a little sugar, pressed into a pan, and baked or chilled until firm. They are also sprinkled on desserts and parfaits as a topping. Honey Maid, for example, sells a 13.5-ounce box of crumbs made from its graham crackers and says one box makes three 9-inch pie or cheesecake crusts. Because graham crackers are made with wheat flour, standard graham cracker crumbs are not gluten-free.
Source: Honey Maid Graham Cracker Crumbs, 13.5 oz product listing (Instacart)
Image: unbaked graham cracker pie crust by little blue hen, CC BY 2.0, via Wikimedia Commons.