Canonical tag

A canonical tag (<link rel="canonical">) declares a page's preferred URL when the same content is reachable at more than one address — with and without a trailing slash, with and without a tracking query string, over http and https.

It doesn't redirect anything; it's a hint that tells a search engine which of several equivalent URLs to index and rank.

Without a canonical tag, a search engine has to guess which of several near-identical URLs is the 'real' one, and can end up splitting ranking signals across them instead of consolidating on a single page.

It's cheap to be defensive about even on a site with no known duplication problem: a canonical tag pointing at the page's own URL costs nothing and prevents a future URL-parameter change from silently creating duplicates.