A canonical URL tag is a way of telling search engines that a specific URL represents the unique copy of a page. Using the canonical tag prevents problems caused by identical or duplicate content appearing on multiple URL’s . Basically , the canonical tag tells search engine which version of URL you want to be appear in google search results.
Does it matter in website google ranking?
Duplicate content is a complicated subject, but when search engines crawl many URL’s with identical or similar content, it can cause a number of SEO problems. First of all, if search crawler/ robots have to wade a through too much duplicate content, they may miss some of your unique content. Second, large-scale duplication may reduce your ranking ability. Finally, even if your content does rank, the search engine may pick the wrong URL as the original. Using canonicalization helps you control your duplicate content.
Problems happens with these URL’S
You might be thinking that canonicalization nothing that you have worry about. The problem is that humans, tend to understand a page as a concept, such as your homepage & For search engines, every unique URL is a separate page. For Example search engine crawlers/robots might be able to reach your homepage in all of the following ways.
For a human , all of these URLs represent a single page. To a search crawlers/robots, every single one of these URL s is a unique page. Even in this example, we can see there are 4 copies of the homepage. In reality, though, this is just a small sample of the variations you might encounter.