The WordPress theme provides a few options for adding links to your site, and customizing how visitors navigate and find information. Refer to the example image and descriptions (screenshot from https://caenwpdemo.engin.umich.edu/block-types/text/) below:
Top Navigation Menu - Navigation of your choosing may be added to the top menu. When the mouse is rolled over a main navigation item, secondary pages will appear and may be selected by the website visitor. Note - The parent menu item cannot be clicked on unless it is added as a subpage (see How do I link the navigation headings on my WordPress website?).
Subnav - Shows children and grandchildren menu items. Grandchildren menu items are not displayed in the top navigation menu, and will only be shown in the subnav if they exist.