P&G
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Layer masks hide or expose the composition beneath them. This enables you to merge images together by masking part of one and exposing that part of the other.
A clipping mask crops an image to the shape of the layer below it.
Aces
For example you can lay a layer mask on a text, and then use black and white to chose how much visible they are. If you fill the mast with black, the image will be invisible, with a white / black gradiant it would fade out.
In short its a way to "mask out" a part of the picture / layer / selection