What are layers? Imagine your image as a photo print lying on the table in front of you. Then put a layer of clingfilm on top of it. That’s a layer. When you make adjustments to your image in Photoshop (or Lightroom, or Elements, or Paintshop Pro), you can then do it on the clingfilm (layer), rather than on the photo print underneath. That way, you’re not changing the original image. But there’s more. If you don’t like what you’ve done on the layer, you can screw it up and throw it away, and it won’t affect the image underneath. You can also adjust the transparency of the layer, so as to vary the intensity of the effect you’re trying to create. Finally, you can have as many layers as you wish, and can also switch layers on and off.
You may work with a number of layers. Each one increases the size of the image file. When you're sure you've finished, go to "layer" in the menu bar, and choose "flatten image". All your edits are then flattened down to just one layer.