The Recently Used Toolbar displays icons for the most recently used items according to your GNOME ~/.recently-used file. The Desklet displays a configurable number of icons in the last used order. As you scroll over the icons in the toolbar they expand similar to the starter bar desklet. If the file has a thumbnail already created, then the thumbnail will be shown instead of the MIME icon. If you click on one of the icons in the toolbar it will launch the application that GNOME has associated with that specific MIME type.