Media attachments means they are tied to a page.Typically they would have been uploaded to a specific page while content was being generated for that page. If there is no attachment shown (far right in the Media admin area), then the media will show “Unattached”. If you decide to update a media file with a new one, then all the attachments for that media file will be updated. If on the other hand you reference the file and then update, the link will still point to the original file.