A webpage can be accessed through two or more different URLs. The most common is the www vs non-www URL. For example, http://domain.com/about.htm and http://www.domain.com/about.htm will always (as far as all websites I encountered) lead to the same web page. There are other strings that can cause a web page to have more than one valid URL ("valid" means typing the URL will lead to a specific page) such as session IDs and trailing slash but those will be discussed in some of my future posts.
How Does it Operate?
The tag is part of the HTML header on a web page, the same section you'd find the Title attribute and Meta Description tag. In fact, this tag isn't new, but like nofollow, simply uses a new rel parameter.
This would tell Yahoo!, Live & Google that the page in question should be treated as though it were a copy of the URL www.seomoz.org/blog and that all of the link & content metrics the engines apply should technically flow back to that URL.