[simranbebi]

any visitor are open your website without www. , in this cases search engine automatically open your site with www.

[Davis Joseph]

Originally Posted by steveschmidt85

www.xyz.com
xyz.com
www.xyz.com/index.html

As per user all these URLs are same but google consider all these pages as different pages with same content and it can cause duplicate content issue it is called canonical issue

Nice answer but he does not answer where and how to use this canonical tag

If we have multiple pages with same contents then give reference of the original page in duplicate page by canonical url tag.