The concept of canonical URLs
Canonical URLs have been introduced by the major search engines in 2009 as a concept to help webmasters fight duplicate content on their own websites. Many CMS's used to have issues with multiple URLs for the same content because of the nature of the CMS. This included Joomla, but also Wordpress, Magento, etc. Without canonicals, search engine struggle which URL to choose for their index. Often, they simply include both (probably deluting the value of both), or the less preferred one. Also print-views and email-views can be duplicates of the source URL. This can lead to lower rankings. With the new technique, you can now choose the preferred version of the duplicates for this same piece of content. As a real world example, say you have these 2 URLs in your site: