Canonical tags, also known as "rel canonical", are a way of telling search engines that a specific URL represents the master copy of a page.

They are used to prevent issues caused by identical or "duplicate" content appearing on multiple URLs. Essentially, the canonical tag tells search engines which version of a URL you want to appear in search results. This is particularly useful in SEO to ensure that the traffic and ranking signals are directed to the correct URL and to prevent dilution of these signals across multiple similar pages.