This mode is only available in the Manager and Develop modules. Its button is in the top toolbar, in the middle. When you are browsing in the Browser or Preview mode, you can press Shift+Enter to turn on Full View. To exit Full View, press these same keys again, press Esc, or double-click the picture.
When you are browsing in Full View mode, the program’s side panels and its top bar are hidden, and only the given file or files and information is shown. The information is shown around the preview. To set what information is shown, use 
Preview Options | Information Display Settings.
To un-hide a side panel or the top toolbar in Full View mode, move the cursor to the edge where that bar is normally shown. The Navigator panel, however, cannot be shown in Full View.