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'
));