Direct Answer
A canonical URL (specified via <link rel="canonical">) tells search engines which version of a page is the 'real' one when multiple URLs serve the same content (with/without trailing slash, http/https, www/non-www, query parameters). Use canonicals on every indexable page to prevent duplicate-content issues and consolidate ranking signals.
Voice answer (≤30 words)
“A canonical URL tells search engines which version of a page is the real one when multiple URLs serve the same content.”
Self-referencing canonicals (page X canonicals to itself) are recommended on every page — they prevent edge cases.
Cross-domain canonicals are valid (e.g., syndication partners pointing to your original) but use them sparingly.
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