Canonical URL
A canonical URL is the version of a page you tell search engines to treat as the master copy when several URLs serve duplicate or near-duplicate content, declared with a rel=canonical link in the head so ranking signals consolidate onto one address instead of being split.
Shopify stores hit this constantly. A single product can be reachable through its product URL, through one or more collection paths, and through variant or sort parameters appended to the query string, so the same content lives at many addresses. Left alone, that scatters links, crawl budget, and ranking authority across near-identical pages that compete with each other.
The canonical tag points every duplicate at the address you want indexed, and search engines fold the signals from the others onto it. Shopify sets sensible canonicals by default, pointing variant and collection-scoped product URLs back to the clean /products/handle path, which is usually what you want.
A canonical is a strong hint, not a command: Google can ignore it if your internal links, sitemap, and on-page signals point somewhere else, so keep them consistent. It also does not replace a redirect. If a URL should genuinely no longer exist, use a 301; reserve rel=canonical for pages that need to stay live but should not be indexed separately.