canonical issue arises when 301 redirects are not properly in place. This means that your website can be accessed by search engines from several different URLs. This means that search engines can then potentially index your site under different URLs, meaning that it will look like a site of duplicated content. Check in Google first and then if you didn't found answer ask here
The issue with canonicalization is determining which URL is the important one with multiple versions of the same content.
Do you means in SEO? Canonicalization is the process of picking the best url when there are several choices, and it usually refers to home pages.
If there are different url of a particular site home page with the same content, we have to redirect that those urls are same. This proces is canonical issue. Can give code for canonicalization redirects at the head portion using meta tags. So search engines will consider those different urls as same.