Mask is an invisible part of the image that determines how the effects will be applied to the image.
The purpose of mask is for special adjustments that may be difficult with selections, for example we want to create mask from luminance.
It is similar to the selection with the difference that we can later process the mask with the adjustment tools or paint into the mask with brushes.
Mask affect future adjustments, but not brushes.
once we created mask we can switch to it by using the menu Image - Show Channel - Mask, and then switch back to Composite.
We can create Mask various ways: Empty mask where we will paint with brushes, From Selection or from Luminance.
White pixels on mask - this is where the image is most affected by future adjustments
Black pixels on mask - this is where the image will not be affected by future adjustments
When Mask is created we can adjust it with any other tools and also apply special Mask only tools to Shrink or Expand the mask.
Shrink Mask - all white areas will shrink by the number of pixels
Expand Mask - all white areas will expand by the number of pixels