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