Questions about masks; in this context they are a way to control and manipulate the visibility of specific areas of an image. Ask anything related to using masks in graphic design software.
Masks in image editing software are a way to control transparency of specific areas of your image. A mask consists of a grayscale channel, called an alpha channel and is often displayed as a ruby overlay so the underlying image can be seen through the mask. The darkest areas of a mask are the most and the white areas are opaque. Shades of gray represent areas of partial opacity that corresponds with the level of gray.