Traybakes are cakes done in a large flat pan, like the one that comes with the oven, and then cut into little squares, rectangles (or anything more creative!)
Chocolate caramel shortbread, fruity cakes, thin sponge with a lemon icing (frosting), flapjaccs, that sort of thing ... I think the American Girl Scout cookies/bake sales are a bit like that aren't they? recipes baked in obe big pan then cut into slices? One of the easiest ways to make teatime treats anyway!
Helen