Masking is the opposite of a stencil-- instead of being a negative image, it is the positive image or a solid image. A mask is a shape placed onto your project then using a colour medium..you apply it to the outside area and the edges (and inside area's if the mask has some) . Once you remove the mask, the colour or pattern underneath the mask remains in the shape of the mask while the outer edges are coloured.
A number of manufactures produce masks in cool designs, but you can make your own as well. If you have a die cut machine you could punch out a shape and then mask it onto your projects. Once you've used the mask, it'll be covered in cool colours that you could then in turn use on a different project.
You could spray over the mask with something like Tattered Angels Glimmer Mist or use a small sponge and sponge paint over the mask. Other mediums you could use are inks, watercolours, spray paint, acrylic paint, walnut ink, bleach mist or even VersaMark to ink the eges then heat emboss.
I have to admit, I have only ever masked I think once since I started scrappin and until I read an article, I never once thought about using punches as masks. I have quite a few punches and most of the time they stay in the drawer under used...so I'm going to give this a shot and see what I come up with!
Why not give it a try too :)