Indexing of a website depend on your website health and various http request response .This is very important in search engine marketing.
It checks for various errors like
1. 200 OK
2. 301 Moved Permanently
3. 302 Found
4. 304 Not Modified
5. 307 Temporary Redirect
6. 400 Bad Request
7. 401 Unauthorized
8. 403 Forbidden
9. 404 Not Found
10. 410 Gone
11. 500 Internal Server Error
12. 501 Not Implemented
For more detailed information i'll recommend you to my source >>>
http://www.seoconsultants.com/tools/headers.asp