In full-screen mode, the window is maximized, and the frame, menus and toolbars are hidden, so that the Whorld image fills the entire screen. Whorld's dialogs remain visible however, so it's possible to edit a patch or playlist in full-screen mode. Note that full-screen differs from Veejay mode, which is similar but optimized for live performances.
To enable/disable full-screen mode, use Window/Full Screen or F11.