Canonical url is the most important for SEO, and is the link pointing to original source. Let say, we write a blog, and share multiple times, or let say we share on multiple blogs. But we keep the same URL for all, the link works as a canonical URL.
Simply, saying it helps finding and identifying unique content. It also helps with duplication, and let search engines know of unique and original content.
One of the SEO techniques known as On-Page activity is Canonical URL. A canonical tag (also known as "rel canonical") informs search engines that a certain URL is the master copy of a page. It prevents content duplication that leads to numerous URLs. In brief, two URLs are produced for the same page, and it is important to tell the search engine which is the correct URL and which is the duplication.
Canonical URL or Canonical Link is the same thing, which is used in HTML. It is used as SEO, where by using Canonical Tag, if the same page in our website is opened with a single URL, then we tell which URL is the most, so that there is no duplicate content in the search. Be visible and get benefit in ranking.
What is Canonical issue?
A canonical issue arises when 301 redirects are not done properly. This means that your website can be accessed by search engines from many different URLs. This means that search engines could potentially index your site under different URLs, meaning it would look like a site of duplicate content.