I'll add my 2 cents worth here. Layer masking is really quite simple and very powerful. I have shown a layers panel with an explanation of the various icons.
I just grabbed a quick image to show you an example. In the below example I wanted to lighten up the left side of the dogs face. On the bottom most layer I made the dogs face lighter but the rest of the image was overly light. In the top most layer I had the dog all correct except for the left side of the face. I created a layer mask and then selected a soft brush and painted in black over the area that I wanted to show through from the bottom layer. You can see the black spot in the layer mask. That represents the area that is showing through from the bottom layer.
Black conceals, white reveals, is a good rule of thumb. When painting you can also adjust the brushes opacity level to attain different results.
There are many online tutorials that are probably better than this but this was done in a few minutes to illustrate it quickly for here.