Canonical issue is the duplicate URL issue. If your website open in both http://yoursite.com and http://www.yoursite.com, then both of the URL will extract the same content, which is canonical issue. The solution of the canonical issue is that you may use canonical tag to tell Google to crawl and index one of the version of your website www or non-www. Other solution is that you can use 301 redirection to redirect your URL to only one URL