Canonical tag

A canonical tag is a line of HTML in a page's head that names the preferred URL for that content. It's used when the same or very similar page can be reached at several addresses, such as with tracking parameters, filters, or both http and https versions.

It's a hint, not an order. Google may choose a different canonical if signals disagree, for example if your internal links and sitemap point to another version. Consolidating duplicates helps avoid splitting links and signals across several URLs.

Practical point: make every indexable page point to itself with a self referencing canonical, and point true duplicates to the main version. Don't canonical to a page that redirects, is noindex, or has different content. Use the URL Inspection tool in Google Search Console to see which canonical Google picked.