The Crumb Layer is a thin layer of frosting you put on your cake to hold the crumbs in so they don’t show up on the surface of the cake.
For the crumb layer, first make sure the cake is cool before you frost it. Brush all loose crumbs off. Then apply a very thin layer of buttercream frosting, pressing your spatula firmly against the cake. Don’t worry if there are some crumbs in this coating. It is called a crumb coat and is intended to hold the crumbs on the cake.
Put the cake in the refrigerator or freezer until the crumb layer hardens. When it is dry, apply your regular frosting. Don’t let the spatula touch the cake, only the frosting. Once frosting is applied, smooth it out as desired.