Adding a 'Drop-down Menu' is a good way to group menu items. For example, a “Genre” collection. A 'Submenu' is a good way to group different collections. For example, adding an “Action” or “Comedy” collection in the “Genre” menu item.
Go to the ‘Content’ tab then select ‘Navigation’.
Select ‘Add a Menu’ and add the name of the menu item, e.g. "Genres", and click ‘Add’. You can also do this to add the 'Submenu' items.
Add the 'Submenu' items you have created by dragging and dropping them under the menu item you added above.
Once done, select ‘Save'.