Fills an area with a gradual transition between one or more colors. This box shows you what colors are selected for the foreground and the background. If you double click you can select what color you want. The two-way arrow allows you to switch the foreground with the background colors. Quick Mask mode lets you view a mask and the image simultaneously.
Color differentiates the protected and unprotected areas.
Quick Mask mode is useful for creating and editing temporary masks. You can start with a selected area and then use Quick Mask mode to add to or subtract from it to make the mask. Or you can create the mask entirely in Quick Mask mode. Once you leave Quick Mask mode, the unprotected areas become a selection. These buttons changes the window modes.