An orphan page is a page on your website that no internal link points to. No menu item, no text link, no breadcrumb. Visitors and crawlers can only find such a page through a sitemap, a link from another website or by already knowing the address.
Why does it matter?
Google uses links to find new pages and to understand how relevant a page is. Google therefore advises that every page you care about has a link from at least one other page on your site. An XML sitemap helps a search engine discover a stray page, but says nothing about where that page sits in your site.
Example
Say a landscape gardener creates a landing page for a spring offer. The page is live and in the sitemap, but once the offer ends the link disappears from the menu. The page still exists, only no route leads to it anymore. A visitor looking for “garden maintenance subscription” will never reach it through the site itself.
Common mistake
Creating links that crawlers cannot follow. Google can usually only read a link if it is an <a> element with an href. A button that only leads to another page through JavaScript does not count as a link.
How do you check it?
Compare the URLs in your sitemap with the pages you can reach by following links from the homepage. Anything in the sitemap that is not linked from anywhere is an orphan page. More checks are in the technical SEO checklist.