URL canonicalisation is where you have a website with different URLs outputting the same content. When search engine spiders see all this content that is the same they can get confused as to what page to display in search engine result pages. The following URLs, although they are different, actually produce the same content.
The way to solve this issue is to redirect any requests to a single page using mod_rewrite. Add a .htaccess file to your root directory and include the following line to turn on the engine.
The following rule will redirect the www page to the non-www page.