Canonical URL helps the webmasters to eliminate the need to duplicate the entire content in the index as it is already supported and listed by the search engine. It is important to understand how canonical URL operates. This tag is included in the header part of the HTML. In many ways, it is quite identical to the nofollow tag. Just as nofollow, it makes use of a new rel parameter.
The above command would tell the search engines that the concerned page should be treated as a copy of the link given in the command, thus eliminating the need to copy the entire content in the new page.
This tag can be used in a variety of applications, such as
When search engines are confused amid the ranking of various versions
When the search engines aren’t guided properly whether to direct the entire content in one search result or separate the link metrics in several pages.
When search engines are not aware which versions of a website to include/exclude.