A canonical URL is your vote for which address is the real one. The same page can often be reached in several ways: with and without www, with tracking parameters, through a filtered view or on both HTTP and HTTPS. Without guidance, search engines have to guess which version to show and may split signals between them.
You declare it with a canonical link tag in the page's head, for example pointing the parameter version back to the clean version. It should point to a live, indexable page, be written as a full absolute URL and be consistent with your XML sitemap and internal linking.
It is a strong hint, not a command. Search engines can choose a different canonical if your signals conflict. Common mistakes are canonicals pointing at redirected or noindexed pages, and pages that all canonicalize to the home page.
Related: Technical SEO, Indexing, Search Engine Optimization.