Think of layers as rectangular transparent plastic sheets. Now imagine...
First, get a sheet and color it all white. Put your sheet down. This is the same as making a layer and filling it with white for a background.
Get another sheet and draw something on it, lines only. Put the sheet down on top of the white sheet. This is the same as making a new transparent layer above the white background and drawing something on it.
Get the idea? Layers = plastic rectangular transparent sheets that you can draw whatever you want on. Things on the layers above overlap the things on the layers below.
*Note: Actually sai may already make a white background layer, and a blank transparent layer above that automatically. No matter, just understand how layers work.