Selection mode/Quick Mask mode button

This button appears directly below the foreground/background color swatches. The default mode for this button is the icon on the left. Whenever you have a selection active, this will display an active selection using marching ants to define the boundaries of a selection. If you have a selection active and you click on this button it will change the selection view to quick mask mode, where the selection will be displayed as a semitransparent colored ‘Quick Mask’. Double-click the button to change the default overlay mask color. Hit to toggle between these two modes.