The Layout Toolbar allows you to switch between the Image or User Interface (Interactive) display layouts. Clicking the buttons will reconfigure the image viewer according to the display layout ascociated with that button.
The Image display button will reconfigure the image viewer to display your image with the palettes and toolbars hidden.
The UI (Interactive) display button will reconfigure the image viewer to show your image with just the toolbars.
The UI+
will show your image with all palettes and toolbars.