A canonical tag, <link rel="canonical" href="..."> in a page's HTML head, tells search engines which URL is the preferred version when several pages carry identical or near-identical content. It's a hint more than a hard directive; search engines usually follow it, but can override it if other signals point strongly to a different page as the better one to index.
Duplicate or near-duplicate content shows up more than people expect: a product page reachable through several category paths, a page with and without tracking parameters, or printer-friendly and mobile versions of the same article. Without a canonical tag, search engines have to guess which version to index and can end up splitting ranking signals across all of them instead of consolidating on one. Every version of a duplicated page should point its canonical tag at the same preferred URL, including that URL pointing at itself, so the whole set resolves to a single, unambiguous target rather than a tangle of cross-references.
