Something put in a thin coating over something else is a layer. Many different kinds of cakes and sweets have layers of different colors or textures on top of one of the other. A cake might have a layer of sponge, then a layer of chocolate filling, then another layer of sponge and then a layer of icing around the outside. The cake in the photo has a lot of layers.
You can also layer your clothes by wearing clothes over the top of each other to form layers.