The Assets tab provides a backend view of the hierarchy of your site. When you click on the tab you see the cards for all of the Sites on your intranet. Each card shows you the name of the site (1) and a description if available. The card also displays icons for the home site (2; triangle), whether it's in the menu (3), and whether it's included in the global search (4).
When you click on a Site card, you'll see the same information as on the initial screen, as well as the number of Apps associated with the Site (1), and the number of Pages associated with the Site (2). Clicking one of the + signs will start the process to add either an App or a Page.