Hi Sophia. Masking is when you cover an image, or portion of an image and then stamp. The mask prevents the ink from covering the first image and makes the new image look like it is behind the first. It is a great way to create depth in a scene. If you look at this post on my blog, you'll see the step by step process I used to mask the images on that card.
To create a mask, I stamp the image on scratch paper and cut it out. Then I use re-positionable mono tape to hold the mask in place.