What Is a Canonical Tag and Why Does Every Page Need One?
A canonical tag is an HTML element placed in the head of your page that looks like <link rel="canonical" href="https://example.com/page" />. It tells Google which URL is the single preferred version of that page that should be indexed and ranked.
Without a canonical tag, Google encounters the same or very similar content at multiple URLs (with www and without, with HTTP and HTTPS, with trailing slashes and without, with query parameters added by analytics or ads) and must decide on its own which version to index. Google often guesses wrong. A canonical tag removes all ambiguity and consolidates all ranking signals to the one URL you actually want to rank.
The Self-Referencing Canonical Rule
Every indexable page should have a canonical tag pointing to itself, even if you think there is only one version of the URL. This is called a self-referencing canonical and it is the default correct state. It prevents parameter-appended versions of your URL (like ?utm_source=email) from being treated as separate pages by Google.