A canonical URL tells search engines which address should be treated as the main version when similar pages have more than one address. It is like marking one copy of a document as the official copy.
The same page might be reachable through links with extra tracking details. A canonical address helps show which version should represent that content in search results.
It is a hint for search engines, not a button that forces them to choose that address. Website owners should use one clear address where possible and make sure the chosen version really contains the page.