Well, if the term "mask" is what's confusing you (and who could blame you), replace the word "mask" in your mind with "transparency", because that's exactly what a image masking does. It allows you to control a layer's or image's level of transparency
. That's it, that's all. There's nothing more to them than that. Now, you may be thinking, "But... I can already control the transparency level with the Opacity option, can't I?", and yes, you certainly can. The Opacity option in the top right corner of the Layers palette also allows you to control a layer's transparency.
But here's the difference. The Opacity option changes the transparency level for the entire layer at once
. If you lower the Opacity level down to, say, 50%, the entire layer becomes 50% transparent. That may be fine for some situations, but what if you want only part
of a layer to be transparent? What if you want the left side of the layer to be completely transparent, the right side to be completely visible, with a gradual transition between the two through the middle of the layer? That's actually a very common thing to do with a layer in Photoshop, allowing you to fade from one image to another. But you can't do that with the Opacity option since as I said, it's limited to controlling the transparency of the entire layer at once. What you would need is some way to control the transparency of different areas of the layer separately. What you would need is Image Masking. For more : clippingcreationsindia.com/