When adding items to a menu, WordPress allows a description to be entered for each menu item. This feature is hidden by default, since many themes don’t display the description on the front end. The beginning of the Displaying WordPress Menu Item Descriptions article explains how to toggle this WordPress setting.
I’m not sure the best way to handle the display of menu item descriptions within a theme, so this is still an experimental feature. Currently, menu item description support is limited to the Custom Menu widget. To enable this feature in the Custom Menu widget, uncomment the following line in the includes/settings.php file:
add_theme_support( 'menu-item-descriptions' );
This will add a checkbox in the Custom Menu widget for toggling the display of the menu descriptions.