A canonical tag tells search engines what page it should display in search results. This means that if you have two pages with duplicate content, adding a canonical tag will tell the search engine which is the master copy and what page is the clone/duplicate.
The canonical meta data is used as a hint to search engines as to what page is the master. Search engines use this to update their index to reference the master copy as the primary copy, and when they deliver search results they deliver the page they believe is canonical.
canonical tag is used for remove duplicate urls such as abc.com & www.abc.com both url open same website but google understand its two different site with duplicate content and decrease your ranking.
Using canonical tag we state craawler to index a particular url.
A canonical tag is to tell search engine that a particular URL represents the replica of a page. Using the canonical tag prevents troubles because of identical or duplicate content material performing on a couple of URLs.
A canonical tag or rel canonical is a way of telling search engines that a specific URL represents the master copy of a page. Using the canonical tag prevents problems caused by identical or "duplicate" content appearing on multiple URLs. This is also a way of avoiding penalties from Google.