As per Wikipedia, In computer science, canonicalization is a process for converting data that has more than one possible representation into a "standard', "normal" or canonical form. (Click Here)

Now if we look at it in SEO terms, this "more than one possible representation" can be replaced with multiple URLs. In SEO terms, canonicalization means redirecting to a single URL which has the highest authority out of multiple URLs.

Sample Code:

<link rel="canonical" href="http://www.yourwebsite.com/" />

What is a Canonical URL?

If I put it in simple words, if you have a same piece of content on multiple pages then search engines would be confused about which version is the best to show to the searchers. This is where the canonical URL comes into picture. If you won't put the canonical URLs to the copy pages then it may get caught in duplicate content penalty. So to avoid this complication, canonical URLs comes into picture, choose the canonical URL of the original page content and put it on other copy pages. Problem would be solved.

Example:

As you can see in this example, there are different versions of home page so in this case, you should set the canonical URL as:

•  https://www.yourwebsite.com

and put it on other versions of home page.

• https://yourwebsite.com
• https://www.yourwebsite.com/index.html
• https://yourwebsite.com/index.html

By putting a canonical tag, you would supply all the link juice to one original page rather than getting it distributed to the multi variants of the same page.

To get a hang of real time "canonicalization" issues, please look at this response of mine that I have put while answering a question about "how to implement canonical URL on various versions of same page?" in MOZ Q&A.

I hope this is a good info that I am sharing today. Please subscribe to my blog if you find it interesting and please do share your views and doubts in comments. I would be happy to discuss with all you smart souls out there.