Add drop-down lists to your Navigation Menu with the help of Menu Folders. All child items nested under a Menu Folder in the Site Builder appear in a drop-down menu when you click the page’s name in the Navigation Menu.
Menu Folders are only used to add structure to the Navigation Menu: they don’t get their own page on your site. If you want a landing page for the content you grouped together, use a Refined Page instead.
...
Add a Menu Folder to your site
...