product.php?item=swedish-fish&trackingid=1234&sessionid=5678The point is that it is better for search engines to not have many links pointing to the same page and in this example allow a header providing metadata to search engine indexers to use a shortened and common URL like
product.php?item=swedish-fishAgain using Google's example, here is the addition to a HTML head element to make this happen:
<link rel="canonical" href="http://www.example.com/product.php?item=swedish-fish" />This obviously saves the search engine companies resources (money!) and makes the user experience better by not having to wade through multiple search results referring to the same page. Of course, search engines try to remove duplicates for you anyway.
Other big winners, if this technique becomes widely used, will be small custom search and semantic web applications that will also benefit from reduced indexing costs.