What is it?
Whenever there is a page on your site that is submitted to Google but has no links to it or from it to anywhere else on the website. Sometimes seen on Google-Search-Console as crawl issue of no-index and no referring page.
Types that are a problem
Any page that you no longer want customers or visitors to see. For example a product that is no longer sold on your site and you removed all links but haven’t deleted the page.
Orphaned pages that are completely fine
Landing page: A page that is meant to keep the customer focused on an action, message on that page.
How to fix
Add a no-robots meta-tag to that page. Click to read the Google guide: