If you have www and non-www URLs, search engines can’t decide which one to show in search results. This is because of duplicate content,even though both URLs are technically different and It affects your site ranking as the popularity of content splits between multiple URLs having same content.
To get rid of this, you should tell Google or other search engines which is preferred URL(out of multiple URLs) to show in search results. So all other URLs redirect to your preferred URL, and thus you can improve link and ranking for you site content. No split of popularity of your content among different URLs with duplicate content.
The Canonical URL is nothing but your preferred URL. When you canonicalizes a URL, search engines pickup it as best choice to represent the content from among other URLs.
Below I explained how to set up canonical URL on Plesk control panel.
How to set up Canonical URL on plesk
1. Login into your web hosting account (Ex: GoDaddy login)
2. Goto Web hosting
3. Click on “Launch” button corresponding to the site which is hosted with Plesk. A webpage opens and shows the list of websites hosted with Plesk.
4. Each site has “Hosting settings” link beside the domain name. Click on it. A new page opens up and shows Hosting settings for that site.
Now go and check whether you fixed Canonical URL or not at http://seositecheckup.com
Note: Enter www URL, if you have set non-www as Canonical URL and vice-versa in the above SEO check site.