A canonical URL signals to search engines which version of a page should be indexed and ranked when the same content is reachable at multiple URLs. Set with <link rel="canonical" href="https://example.com/page"> in the page's <head>, or via the Link HTTP header for non-HTML resources.
Common cases that need a canonical: pages reachable with and without trailing slash, www and apex variants, HTTPS and HTTP, paginated lists (canonical to page 1 or self-canonical depending on the use case), session-ID query strings, UTM-tagged URLs from email campaigns, and a single product appearing under multiple category paths.
A self-referential canonical (every page declaring its own URL as canonical) is the safe default and protects against accidental URL parameter explosion. Cross-domain canonicals are honoured by Google but should be reserved for republished content where you control both domains.