When activated on the Admin Features page, a Fullscreen link enables the site user to see the center part of the page, without the top and bottom bars or side columns, displayed full window size. This is useful for viewing large images, wide tables, etc.
The feature causes an icon (a rectangle with an upward arrow) to be displayed, usually in the upper right region of the screen. Clicking on the icon causes the middle part of the page to expand over the parts which usually show the side modules. The exact location and appearance of the icon is dependent upon the theme being used.