What is a mask?

A mask is a layer.
A greyscale, raster, layer.
(Greyscale means exactly what it sounds like - you are only able to paint in black, white or grey. Raster means an image constructed from a grid of pixels, like .pngs, .jpegs, and .bmps.)

A mask can be opaque or transparent.
It hides (masks), and/or allows to be visible, portions of the layers underneath.

Black is totally transparent and allows layers to be visible through the mask.
White is opaque and hides underlying layers.
(Both these colours are "virtual" - you paint with "transparency" or "opacity", but for simplicity they are called "black" and "white".)

The original "background" image of a raster file can't have a mask in GIMP. If you want it to, you must copy it (and then delete the "background" layer if you don't need it anymore).

When you add a mask to an individual layer, from that point on every paint-stroke will be in black, white, or shades of grey, on the mask itself. You will not be able to paint on the underlying layer again until you "Apply Layer Mask".

Check out this page at sourceforge for a rather brilliant tutorial and introduction to masks.


Why do you care?

You can use GIMP very happily and never use masks. However, they can be very powerful tools for image manipulation. Try them.