Enabling Full Screen allows users to view your SmartFrames in a full-screen mode. In the Admin Panel you can select whether you want to add a Full Screen button (recommended) or open the full screen mode upon clicking on the image. Keep in mind that if the latter option is selected, Deep Zoom or Image as Link (if used) will be switched off.
You can preview the Full Screen functionality here:
In order to preview how the Full Screen will work with an actual SmartFrame, you can click on the Preview button at the bottom of the page. It will open a full preview of the theme, including the changes you’ve just made, even before you save them. Once you’re happy with the results, you can save the changes by clicking on the Save button.