Using Quick Mask mode, you can modify (or create) a selection using pixel-editing tools.
When entering this mode, your selection is temporarily presented as a pixel mask. Pixels can be added or erased using the standard Painting and Erase tools.
By default, the Quick Mask is presented in the workspace as a translucent, monochrome overlay. The monochrome areas are not included in the selection. This default view can be changed depending on your preferences.