Canonical URL
A canonical URL is the address a search engine treats as the primary version of a set of duplicate or near-duplicate pages. Your rel=canonical tag is a hint that is weighed against redirects, internal links and sitemap entries; the engine makes the final choice.
The same content is usually reachable at more than one URL: with and without
www, with and without a trailing slash, with tracking parameters appended, or
through a print or AMP variant. A search engine groups these into a cluster and
picks one to index and show.
Your <link rel="canonical"> tag declares your preference. It is a strong hint
and it is usually respected, but it is not binding. The engine also weighs which
URL you link to internally, which one your sitemap lists, which one redirects
point at, and whether one is HTTPS.
When Search Console reports “Duplicate, Google chose a different canonical than user”, the engine has disagreed with your tag — almost always because your other signals contradict it. The fix is to make the signals consistent rather than to repeat the tag.
A canonical is not a redirect. It leaves both URLs reachable and simply
expresses a preference. If a URL should genuinely no longer be served, a 301
is the correct tool.