Relationships describe which other pages reference a page or file within the CMS. To check relationships, click on the tab in the right of the uppermost bar.
If the page has no relationships, it will simply say so. However, if there are other pages within the CMS which link to that page, it will list them. If you at any point need to unpublish or rename a page, you should first check its relationships. If there are any relationships, you need to make sure to go to those pages and edit the content there to either change the destination to the new name or remove the link entirely.