Go to "Appearance > Menus" and at the top locate "Screen Options" and click it.
Check the box beside "Description" to enable this option.
Now go to each of the menu items, expand them by clicking the little arrow.
Inside each of the menu items, you can add a description for your menu items.
Make sure to save your changes afterward by clicking the "Save Menu" button.
Beneath here is a video tutorial on how to create and manage your navigation.