I had to get good at photoshop since I'm a photographer. I'm a nurse by profession, but a photographer on the side, so a good working knowledge of PS is essential for me. Layers are actually really simple and I just read the tutorial on CAW and if you understand layers in photoshop (or any of the imaging programs) it makes it much easier to understand layers in CAW. My best advice is to google "understanding layers" and there are numerous articles and youtube videos on the topic.
But basically......think of layers as just that.....LAYERS. Imagine your bed with a sheet, (layer 1) a blanket, (layer 2) and a bedspread. (layer 3) If you had a transparent bedspread, you'd be able to see the blanket below it. If it was just kinda see through, you might be able to kinda see the blanket below it. That's the opacity of the layer......the amount that the underneath layer shows through to the above layer. When you're working with terrain paints, the higher you set the opacity, the less you will be able to see the layer beneath it. The lower you set the opacity, the more the underneath layer will show through. When working in CAW, the layers allow you to easily adjust the terrain with the different terrain paints and objects such as trees. For example, you can set the trees all on their own layer so the game will run faster. It also will allow you to easily delete that entire layer if you decide you don't want any trees in that location. (like taking the bedspread off the bed) Or you can think of it as making a multi-layer cake with the decorations being the trees or other objects and the frosting being the terrain paints.
Hope this helps a little.
Once I get into CAW more, I'll see if I can explain it a little better.