Reply To: What is canonical tag in HTML

Canonicalization is a concept which could be understood by the following example:
For an instance if a website open in both ways in a browser:
Example.com
http://www.example.com
then the search engine consider it as duplicate web page of each other, so yes it is relevant from the SEO point of view, because Google hate duplicate content.
By adding canonical tag in your website with that particular URL, is a signal to Google, which should be picked for the crawling. Only the canonical link get crawled while another URL will be ignored.