Layers: The Basic Concept
Imagine taking two photos and laying one on top of the other. You can't see the one on the bottom, of course. But suppose that you could make the top photo somewhat transparent, so that the other photo showed through. That, in a nutshell, is the concept of layers.
In most photo editing programs, you can add as many layers as you like, and you can vary the opacity of each layer: The lower the opacity, the more readily visible the underlying layers will be. A layer can be composed of almost anything. You can layer two different photos, or two copies of the same photo. A layer may even be a solid color. You usually control your layers by working with a Layer Palette, such as this one in Adobe Photoshop Elements.