Definition: a thin first layer of buttercream over a cake that traps loose crumbs, so the final coat goes on clean.
The crumb coat is the trick behind a tidy iced cake. You spread a thin, scrappy layer of buttercream all over the stacked sponges — itwill pick up crumbs, and that’s the point — then chill it for fifteen minutes or so until set. That firm layer locks the crumbs down, so your proper final coat glides on smooth and crumb-free. Skip it and you’ll be chasing crumbs through your lovely top layer.
In practice: thin coat, chill to set, then apply the final layer over a firm base.