The quick mask is a way of selecting parts of an image using the paint tools. A good way to use it is to first create a rough selection using the Scissors or Free Select tool. Then start using the Quick Mask:
To activate the Quick Mask, in the lower left corner of the image window, click the icon with the dashed box. The Quick Mask icon now changes to a red box.
The Quick Mask highlights the deselected parts of the image with a red overlay. Areas appearing in their normal color are selected.
Note: Changing the Color of the Mask
To use a different color for displaying the quick mask, right-click the quick mask button then select Configure Color and Opacity from the menu. Click the colored box in the dialog that opens to select a new color.
To modify the selection, use the paint tools.
Painting with white selects the painted pixels. Painting with black deselects pixels. Shades of gray (colors are treated as shades of gray) create a partial selection. Partial selections allow a smooth transition between selected and deselected areas.
When you are finished, return to the normal selection view by clicking the icon in the lower left corner of the image window. The selection is then displayed with the marching ants.