What are canonical tags ?

A canonical tag simply tells a search engine that the original version of a page is at a certain address.

This is the syntax;

<link rel=“canonical” href=“https://example.com/sample-page/” />

It goes in the head of your HTML document.

If you had a page that was accessible by several different URLs (faceted navigation for example) but basically has all of the same content, you can designate the original or main source of that content with a canonical URL. Google can sometimes chose a different canonical but usually doesn't.

Here is a great blog post by ahrefs on the topic.

https://ahrefs.com/blog/canonical-tags/

A canonical link (also called canonical tag, canonical URL or URL canonicalization) is included in the HTML code of a webpage to indicate the original source of content. This markup is used to address SEO problems with duplicate content which arise when different pages with different URLs contain identical or nearly identical content. This duplicate content problem can arise in a number of ways, often without the website administrator’s knowledge, including when:

• a webpage is accessible with or without “www” prefix in its URL
• a webpage is accessible via “HTTP” and “HTTPS” protocols
• there are multiple versions of a webpage using different URLs (for example a print version or when using sort categories etc.)

• For SEO purposes, the canonical link shows Google and other search engines which URL corresponds to the original source of content and should be shown in search results. It is added as a meta tag to every URL version of a given webpage and indicates the canonical URL. Note that the link is only a recommendation; Google can ignore it if it is used inappropriately.

  Ideally, every page of a site should contain a canonical link to avoid the risk of duplicate content in the future.