Drop down menus: Parent Pages/ Sub Pages

Hi

Just wondering how you create drop down menu’s from parent pages to sub pages?. If you look at the website https://www.ncptt.nps.gov/ and if you place the cursor over the “About” page for example, it will give you a drop down menu and allow you to select sub pages such as “Contact” and “Staff” etc which are listed under the parent page.
Just wondering how these drop down menus are created?
Thanks in advance

Regards