What a Mask Does
Let's start with the basics. A mask is a technological way of doing what designers have done for years, hide parts of an image by putting something in front of it. Where pre-digital designers might have cut a hole in a piece of card and put it in front of a photo, we use a virtual piece of card that we can change at will. It's a much more flexible way of doing things and doesn't involve precise cuts with sharp craft knives!
Here's an example of a mask, using the Meerkat image, doing something very simple: highlighting his head with a star and hiding the rest of the image.
the first half of the screenshot shows the image before we apply the mask, a white star covers our furry friend. When we apply the star as a mask, the star acts as a window through which our pixels can be seen.