The rel=canonical element which is also often called the canonical link, is an HTML element that helps webmasters prevent duplicate content issues. It does it by specifying what the canonical URL or the correct version of a web page is.
The idea is quite simple, imagine if you have several similar versions of the same content, you can then pick one canonical version and point the search engines at that. This solves the duplicate content problem where search engines don’t know which version of the content to show.
Choosing the canonical url for the correct web version improves the SEO ranking for your site, as the crawler now knows that there is only once correct version and the rest will be treated as links to the link, so you avoid the so called duplicate content.
What is duplicate content?
Duplicate content is when you have the same content present on different pages on one website or multiple websites. Sometimes the duplicate content occurs unintentionally, for example when having a blog, the same post content can be created for the link http://www.studioweb.com.au/create-email-accounts and http://www.studioweb.com.au/tags/emails/create-email-accounts. This can be avoided by adding a canonical url for the former link, so that way the latter link will be just a link which the crawlers can then crawl without affecting the SEO ranking