Descriptions are added via the WordPress core menu system’s menu item settings.

Go to Appearance > Menus.

Reveal the menu item’s settings by clicking the arrow on the right of the menu item

If you do not see the Description setting, you can reveal it by clicking the Screen Options tab in the upper right of your screen, then checking the “Description” box under “Show advanced menu properties”

Enter your description in the description box.

Save the entire menu to save the description.

Your description will appear below the Navigation Label inside the item link.