Masks are shapes that allow you to hide a particular part of the output or even make hollow shapes.
Indeed, a mask affects the group it belongs to, so if you overlap faces from the same group and put a mask in it, you'll just see black where the mask is, but if you place faces from another group below, you'll be able to see through the face from the group with the mask!
Note: masks are not affected by group effects and any overflow from its group will not be displayed inside them, but they can be affected by output effects.
To make a mask, simply draw a face however you want, select it and click on the "Use as Mask" option from the right-click menu or the associated options menu in the layers panel.
For visual instructions, a part of the video tutorial on shape drawing is dedicated to masks!
Note: you can't directly draw a mask, you always have to draw a face first.
To turn a mask back into a face, select it and click on the "Unmask" option from the right-click menu or the associated options menu in the layers panel.
Basic manipulation operations and transformations of shapes are described in a dedicated article.