What is Quick Mask and how does it work?

What will be an ideal response?

Many graphic arts professionals refer to alpha channels as "masks " because the non-selected areas are "masked" by the black areas in the alpha channel. Quick Mask works with the same concept as alpha channels. Quick Mask is a visual aid that gives you a preview of your selection. It helps you to see your selection by "masking out" the areas that aren't selected and showing only the areas that are. You can increase or decrease the opacity of the Quick Mask by double-clicking the Edit in Quick Mask Mode button on the Tools panel. The Quick Mask Options dialog box opens. In this dialog box, you can change the color and the opacity as well as decide whether you want the color to indicate the masked area or the selected area. You can toggle between Quick Mask Mode and Standard Mode by pressing [Q].