If you are not familiar with WordPress menus, please read the official manual regarding Menus here:
https://codex.wordpress.org/WordPress_Menu_User_Guide
Theme menus
This theme has 2 menu locations:
- Primary Menu
- Footer Menu
the menu design can be customized in the Header section.
Primary menu
It appears in the top bar and as the off-canvas menu for mobile. It supports 3 levels of depth.
The main menu can be converted in to an Off Canvas menu also for desktop via Customizer settings. To set a main menu, after creating your menu following the official WordPress manual, go in Appearance->Menu->Menu Locations and set it as main menu:
Footer menu
One level menu that appears in the footer. Doesn’t support sub-menu items. In desktop, its length can be maximum half of the width.