A canonical tag is a line in a page telling search engines which address is the official version, for when the same content is reachable at several different URLs.
Why one page ends up with several addresses
Rarely on purpose. A page can be reachable with and without www, over http and https, with and without a trailing slash, and with any number of tracking parameters attached from campaigns and shared links.
To you that is one page. To a search engine each address is a separate URL showing identical content, and it has to work out which one to index.
What the duplication costs
Mainly it splits credit. If some sites link to the www version and others to the plain one, the value of those links is divided between two addresses instead of accumulating on one.
It also wastes crawl effort on large sites, and it produces the mildly maddening situation where the wrong version of a page is the one that ranks.
How to use it
Every page should declare its own canonical URL, pointing at itself in its preferred form. That single habit resolves most of the accidental duplication a normal site produces, including the parameter variants you never anticipated.
Use absolute URLs, be consistent about trailing slashes and www, and never point a canonical at a page that redirects. Do not canonicalise every page to your homepage, which is a surprisingly common mistake and effectively tells search engines your other pages do not exist.
Where this shows up in the tracker
Every term in this glossary is something the product actually measures or acts on. Plans start at £29 a month, with rank tracking, AI visibility, the daily AI readiness scan, security and domain health, Search Console, backlinks and audits on every one of them.
Related terms
Cannibalisation
Cannibalisation is when two or more of your own pages compete for the same search, so Google keeps swapping which one it shows. Both pages are weaker than one good page would be.
robots.txt
robots.txt is a plain text file at the root of your website telling automated visitors which parts they may request. It is the single highest-consequence file on most websites, because one wrong line can remove you from search results entirely.
Structured data
Structured data is markup you add to a page describing what it is in a form machines can read without interpreting your layout: this is a product, this is its price, this is the company that sells it.
Rank tracking
Rank tracking records where your website appears in search results for chosen keywords, checked on a schedule so you can see movement over time. It answers where you are, and it is no longer sufficient on its own to explain what that is worth.