Publish Related Items are:
Clone references – items that are clones of the selected item.
Media references – media items that are related to the selected item.
Alias references – items that are aliases of the selected item.
This is lifted from the Sitecore Doc site its near the end. Here is the link [Publish an item to your website][1]
Hope this helps. [1]: https://doc.sitecore.net/sitecore_experience_platform/content_authoring/publishing/publish_an_item_to_your_website