Selective Correction with Adjustment Layers and Masks

About the layer mask

Notice the shadows on the buildings are much lighter, but the mountains and the clouds are unchanged. That is because our selection created a mask for the adjustment layer. You can see a black and white thumbnail representing this layer mask in the layers palette. Where the mask is black the adjustment is not shown. You could modify the effect of the levels adjustment by clicking on the mask thumbnail and painting on the document with shades of gray.

Now hold down the Ctrl key (Command on Mac) and click the mask thumbnail on the layers palette. This will reload the mask as a selection.