Layer Masks allow you to hide (mask) part of a layer (or effect) using powerful painting and selection tools.
Think of Layer Masks as a better way of erasing.
It is a way to erase (hide) parts of an image or an adjustment layer without affecting the original photo.
You can add a layer mask to any layer by clicking on the Add Layer Mask Icon.
Paint with black to "mask" or hide or erase parts of an image or effect.
Paint with white to "unmask" or reveal parts of an image or effect.
You can also use the magic wand and make selections.
Just make sure black is your BACKGROUND color before you press DELETE or BACKSPACE.
All Adjustment Layers are added with a Layer Masks attached to them.
Paint Black to Mask or Remove the effect.
Paint White to Unmask or Apply the effect.