Canonical URL: the search engine friendly URL that you want the search engines to treat as authoritative.; In other words, a canonical URL is the URL that you want visitors to see. Quite often canonical URLs were used to describe the homepage.; The typical example used is that most people treat the following URLs as the same: www.example.com, example.com, www.example.com/index.html,etc;
Canonical URL: the search engine friendly URL that you want the search engines to treat as authoritative.
In other words, a canonical URL is the URL that you want visitors to see. Quite often canonical URLs were used to describe the homepage.
The typical example used is that most people treat the following URLs as the same: www.example.com, example.com, www.example.com/index.html,etc
From a search engine perspective, this can cause a bit of an issue. Hence the idea of canonicalization.
Canonicalization is the process of declaring one particular webpage URL to be original referring to multiple duplicate Content/URLs.
Duplicate Content is nothing but having many forms of a single URL, these multiple URLs are generated by web servers, referral websites, etc.
An easy way to get final destination URL is through the canonical tag or by redirecting all of these pages to one authoritative page.
Canonical Tag specifies the original URL to the search engines.
The pages having same piece of content, but different URLs and the links that are supposed to go to the original content, split up.
In SEO, when link building is done; link points to original content and due to duplicate URLs link popularity splits up in duplicate URLs diluting the importance of link building.
Without having a canonical tag, we are increasing the SEO vulnerability of the site as search engines find difficulty to find the original one.
So, having canonical tag would strengthen your SEO and also make the website robust.