Canonical URL
A tag in a page’s code telling search engines which address is the main one when the same content is reachable at several.
Glossary · SEO & AI search
One page often has several addresses: with parameters, with and without a trailing slash, with filters or sorting. To a search engine these look like several pages with identical content.
The canonical tag says which of those addresses is the main one. The search engine then shows that one and credits it with what all the variants have earned.
The tag is a recommendation, not an order: if it contradicts other signals, such as internal links, the sitemap or redirects, a search engine may pick a different address. All of those signals should therefore say the same thing.