When you create an adjustment layer, the effects of the adjustment are applied to the entire image. For example, if you use the Curves adjustment, the entire image receives the effects of the adjustment. It's true you can modify the adjustment with the use of layer blending modes, and opacity settings but, the effects are applied equally to the entire image. The problem is that many times you don't want the adjustment applied to the entire image. For example, color correcting a portion of the image, or lightening the shadows of an image without applying the same lightening adjustment to the highlights. Photoshop handles this problem with the use of masks. When you create an adjustment layer, Photoshop creates a mask with the image. The mask controls how the adjustment is applied to the image, and you control the effect by painting in the mask with black, white, or a shade of gray. When you paint the mask with black, it will mask the adjustment, painting with white fully applies the adjustment. If you paint with 50 percent gray, then 50 percent of the adjustment is applied to the image.
Layers palette.
Paintbrush tool.
Foreground Color box on the toolbox to black as the painting color.
The areas painted black mask the adjustment, and return the image to normal.