Buckle is a thick-batter cake with fruit (usually blueberries) and a crumble topping on it. Some recipes will have you put the fruit on top of the batter, some will have you stir in into the batter. In either case, as the batter bakes and rises, the fruit sinks into it, but depressions remain in the surface of the cake wherever there is a piece of fruit underneath. This “buckled” appearance has led over the years to this type of easy, homey dessert being given the affectionate name of a “buckle.”
Buckles are often used as “coffee cakes”, meaning quick cakes you can whip together when you get word company is coming by in an hour for coffee and you want something to serve.