When writing or modifying a theme, the function wp_nav_menu() displays the contents of menus configured in your settings. If you have a lot of little menus the list can get hard to manage, so this plugin lets you group several menus under a single menu.
To use it, simply add a submenu parameter to the arguments of wp_nav_menu, like so:
wp_nav_menu(array(
'menu' => 'header',
'submenu' => 'About Us'
));