A canonical tag is an HTML link element (`<link rel="canonical" href="...">`) that tells search engines which URL is the master version when the same content is reachable at multiple URLs. It consolidates ranking signals onto one page.
Canonical tags solve duplicate-content problems caused by URL parameters, tracking codes, mobile/desktop variants, http/https mismatches, and trailing-slash inconsistencies. Without them, Google may index every variant separately, splitting link equity and confusing rankings.
The self-referencing canonical (a page pointing to its own URL) is best practice — it explicitly says 'this URL is the master' rather than leaving it ambiguous.
Wrong canonical tags can de-rank an entire section of a site. A homepage pointing canonically at a product page, or every page pointing at the homepage, will collapse the indexed footprint to a single URL. Auditing canonicals on every page is essential.
See it in the product: SEO monitoring.