A subpage is placed under a parent page in the navigation.
Add a new page as a subpage
Go to Pages> Add Pages, then go to Page Content and Options. After entering title and text, you will find the Navigation options at the bottom. Give the new page a menu title and place it in a navigation. Below you can also check: "Show the next level of menu items" so that the new page appears as the submenu on the previous page.
Add an existing page as a subpage
You can also place an existing page in a submenu. This can be done via the back end of the page, or by sliding in the menu structure of the website. Via Page content and options, you can walk the same steps as with a new page. If you go through Structure> Navigation, you can also create a submenu via "Add menu item." Here too you enter a menu title, link it to a page, then drag them to a parent page. It is important that in the back end of the page "Show the next level of menu items" is checked.