example

Before	After Adjustment Layers
In the image above, the sky is slightly washed out and I prefer to have it darker. If I darkened the whole image, I'd be making the mountains in the bottom half too dark and would lose detail. I apply a Levels Adjustment layer (named Levels 1) and adjust the levels to darken the sky. Unfortunately, because the mask attached to this adjustment layer is all white, this effect is applied to the whole image and my mountains are too dark. The top layer (Levels 1) is a Levels Adjustment layer (you can tell by the icon). The attached mask (which is all white) tells Photoshop to apply the adjustment to the whole image. I need to Mask the mountains from this adjustment. I use a large, soft-edge paintbrush set to pure black and paint over the mountains on the mask, which removes the effect of the Adjustment Layer in those areas. This returns the mountains to their original tone.