What Is a canonical tag?
A canonical is also called “rel canonical” is a way of telling search engines that a specific URL represents the master copy of a page. Also the use of canonical in your site is to prevent it from duplicated content.
This is a way of preventing search engines from crawling content from a website that is identical to the content on another website.
If you are not sure whether your website has duplicate content, you can test it on Google’s Search Console here: https://search.google.com/structured-data/testing-tool/
It will tell you whether Google is able to crawl your web page or not.
How to Add Canonical to Your Website
In order to add a canonical tag, you need to add the following code to your header.php file:
Set canonical URLs for duplicate content – Knowledge Base
In addition to that, you need to add a canonical tag to the specific page which is duplicated content. You can add it in the header.php or in the specific page.