Unpublished is media you've uploaded but not associated and published with a blog post. It is not public. By design, you can upload a media file today, schedule the blog post to publish 3 days from now, and until that post is published, the file is not public. Once the post does publish, your server contacts blubrry to move the file to public. This is also helpful if you filled up your monthly quota, you don't have to re-upload the file, you just wait until the space is available then you can publish at that time.