What is a layer mask?

A layer mask is a layer that you put on top of another layer and make a part of it visible and a part of it invisible to the other.
As an example, I have an image with two layers.
The bottom layer (Layer 1) contains a picture.
The top layer (Layer 2) is completely filled with the color yellow and contains the layer mask.
A layer which is provided with a layer mask has two miniature views.
The left thumbnail (1) represents the image in this layer, in this case, this is the yellow color.
The thumbnail on the right (2) represents the layer mask.

If you want to mask off temporarily, click this thumbnail with the Shift key pressed (1). Click again on this thumbnail if you want to make it visible again.
If you wish to display the layer mask in the window, click the thumbnail with the Alt key (2). To return, repeat this operation.
A layer mask is an image consisting of grayscale. This means that it only consists of black, white and intermediate grays.
The part that is colored black is completely transparent, the part that is colored white is completely covered. All intermediate shades of gray are semi-transparent. The darker the gray the more transparent.

By using layer masks, you always have original photo available and foremost, you can modify or delete the layer masks at any time. That is when you save the image in a format that supports layers, for example. PSD or Tiff.
However, if you save the image in a. JPG format, the mask layers are applied, but they will be combined into one layer and they can not be changed later.

Layer masks are one of the most powerful features that Photoshop allows you, depending on your inspiration, you can create the most incredible creations.
In the examples below, you can see that I did not have much inspiration.

Or maybe a little too much.
This is not made with a layer mask.

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