A Menu widget is used to generate multi-level responsive sub-menu.
There are two ways:
To create a page layout with a sub-menu:
'tpl' => 'webparts/navigation/menu_vertical_list.tpl.php',
'starting_level' => '1', 'generation_scale' => WebPartZone::SCALE_CURR_BRANCH
* This apply to the desktop view. On mobile, the menu is hidden by default as all levels are generated by main navigation (hamburger menu).
Next, you need to apply your page layout to your pages that should render the sub-menu.