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 red overlay. The red areas are not included in the selection. This default view can be changed to show the masked area as black, white or transparent depending on your preferences.