The link element rel=canonical in the head of a page with a certain URL points search engines to the original URL of that page.

Some content on a site may be accessible through different URLs. This is confusing to search engines, who may punish the site for presenting 'duplicate content'. From the point of search engine optimization it is therefore good to tell that one of the pages is the original one ('canonical') and others just aliases. This is achieved by code that determines which page is the original and that includes this tag to link the other pages to the original:

<link rel="canonical" href="">