There are seven sections.
Masks are like stencils.
1) Make a stencil by cutting holes in a sheet of plastic film.
2) Put the stencil on a wall.
3) Spray it with paint.
The paint goes where the artist wanted it to go—due to the stencil.
Your "paint" can go where you want it to go on a photograph—with a mask.
Below, the photographer is "spaying" a Levels adjustment layer on her photograph.
The Levels is going through the white part of the mask
The levels isn't going through the black part of the mask.