can any one elaborate this code?

[Gardner]

This is the only thing you need to know about it: (From the very top of the page)

Please note: We are in the process of deprecating the REST API, so if you are building a new application you shouldn't use this function. Instead use the Graph API by issuing a POST request to the tags connection of the Photo object

In other words, don't use this anymore. There is another way to do this now.

Edit:
http://developers.facebook.com/docs/reference/api/

This is the new API. After reading that you should be able to do what you wanted to do.