A layer mask allows you to hide and reveal parts of a single layer. Layer masks, like Quick Masks, can be edited. One advantage of working with a layer mask is that, if you don't like the result, you simply discard the mask, and your image is left untouched. If you like what you see, apply the mask to make the changes.
Not on the Background
To make a layer mask, select an area of the image to mask and click the Add Layer ...