What is a canonical URL?
A canonical URL is the address a site names as the main version of a page when the same or very similar content is reachable at several URLs. It is set with a rel=canonical link in the page head and tells search engines which version to index and credit with ranking signals.
Why duplicates happen
Tracking parameters, trailing slashes, upper and lower case, http and https, print versions and filtered listings can all produce several URLs for one page. Without a canonical, search engines pick one themselves.
How search engines treat it
Google treats the canonical tag as a strong hint, not a directive. It weighs it with other signals such as redirects, internal links and the XML sitemap, so all of them should agree.
Good practice
Give every indexable page a self-referencing canonical with its absolute, clean URL. Use 301 redirects rather than canonicals when an old URL should disappear, and keep hreflang alternates pointing at canonical URLs.
