What is the X-Robots-Tag?
Google introduced the new X-Robots-Tag directive in 2007 to allow webmasters to control access to non-web page documents, such as Adobe PDF files, video and audio files.
The X-Robots-Tag is included in the HTTP header of a document. The HTTP header is the initial reply of a server to a query. It contains information about the document that follows, including content type, creation date, character set, encodings, etc.
The new X-Robots-Tag allows webmasters to add information about search engine indexing to the HTTP header.

Which commands are supported by the X-Robots-Tag?
At this time, the X-Robots-Tag supports the following commands:

~ X-Robots-Tag: NOINDEX (Use this tag if you don’t want to show the URL in Google’s search results)

~ X-Robots-Tag: NOARCHIVE (Use this tag if you don’t want to see a Cache link in the search result pages for the document)

~ X-Robots-Tag: NOSNIPPET (Use this tag if you don’t want to display a summary in the search result pages.)

~ X-Robots-Tag: NOFOLLOW (Use this tag if you don’t want Googe and Yahoo to index the links in the page.)

~ The X-Robots-Tag is currently supported by Google and Yahoo. Other search engines don’t support the tag yet.

How to add the X-Robots-Tag to your documents
It depends on your server and on the method you use to create your web pages how you can add additional tags to the HTTP header. If your web server uses Apache, you can use the htaccess file to modify your HTTP headers.

In general, you shouldn’t mess with the HTTP header if you’re not 100% sure what you’re doing. A broken HTTP header can keep search engines away so that no search engine will index your website.

Instead of using the X-Robots-Tag, you can also add meta tags with the same functionality to your web pages:

You can edit these tags directly in the head part of your web pages. They have the same effect as the corresponding X-Robots-Tags explained above.

Do you need the X-Robots-Tag or the corresponding meta tags?
If you don’t want to limit access to your documents of if you only have normal web pages on your website then you don’t need the X-Robots-Tag or the meta tags.

The X-Robots-Tag has been designed to restrict access to your documents. That means that it does not help to increase search engine rankings.

The new X-Robots-Tag and the meta robots tag allow you to restrict access to your documents.
If you want to make sure that search engines index all of your web pages then you have to optimize your web pages.

Search engine robots must be able to easily parse your web pages and they must be able to find the right elements on your web pages. If you optimize the right web page elements with the right keywords, your web page will get high search engine rankings for these keywords.