Backlinks show you what pages link to the page you are currently viewing. To access backlinks choose the Window ▸ Palettes ▸ Backlinks menu item, or click on the icon in the toolbar.
How do you use backlinks? Sometimes people use this feature when searching for content. You can find the page you are looking for using the search box, and then look at the backlinks to work your way backwards through your document's link structure to figure out where you are.