Canonical URL
In short
A canonical URL is the address Google should treat as the main version when the same content exists at several addresses.
The same page can often be reached in several ways: with and without www, over http and https, or with a UTM parameter on the end. To you it is one page. To Google it is several addresses with the same content, and Google then has to pick one of them to show in search.
How to say which one counts
Google states that a redirect is the strongest signal, that a canonical tag in the code is also strong, and that an address in the sitemap is a weak signal. Google can still choose a different address from the one you suggest. The tag is a recommendation, not an order.
Without clear signals, the wrong variant can end up in the search results, and links and other signals get split across several addresses.
Check a page
Type your address into the browser with and without www. Both should end up at the same address. Then open the page, press Ctrl+U to view the source code and search for "canonical". The address there should be the page's own, without extra parameters. If you have access to Google Search Console, URL Inspection shows both the address you declared and the one Google chose.
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