what exactly is canonicalization in seo?

when the same page can be reached at more than one address, canonicalization is picking which address is the real one. think http vs https, www vs no www, trailing slash, ?utm= versions. the canonical tag in the head says "this one counts". for a migrated site this is the thing to check, because the common mistake is new pages whose canonical tag still points at the old domain, so google keeps crediting urls that now redirect. open view source on a few new pages and look for rel="canonical", it should be the page's own new url. search console's url inspection shows the google-selected canonical if you want to confirm.