Masking is a technique used to selectively hide or reveal parts of a layer or image. It allows you to make specific adjustments or edits to certain areas of an image while leaving the rest untouched.

Think of it as a digital version of a painter's mask, or stencil, which is used to protect certain areas of a painting while the artist applies color or texture to other areas.

In Photoshop, you can create a layer mask, which is essentially a grayscale image that acts as a stencil. Areas that are white in the layer mask are visible, while areas that are black are hidden. By painting with shades of gray, you can create varying levels of transparency or opacity, allowing you to blend different layers or elements together seamlessly.

Layer masks are non-destructive, meaning that they do not permanently alter the original image. If you make a mistake or want to adjust the mask later, you can simply erase or paint over it as needed.

If you have a multi-layer image it is usually critical that the order of the layers (the "stack") are selected carefully as the mask only applied to the layer it is on.  NB You can "collect" several layers together into a "group" and then apply a mask to that group which will affect all the enclosed layers.

Overall, masking is an essential technique that allows you to create complex and detailed images with precision and control.