A Clean URL is a URL devoid of any extra query strings. For example, the URL, http://www.example.com/index.php?pg=contact would be considered an "unclean" URL due to the "?pg=contact" at the end of the URL. This can affect the spiderability of your website by the search engines. A clean URL, would look like this: http://www.example.com/contact, and is without an unnecessary code.