A product page reachable with and without a tracking parameter, a listing sorted three ways, an article available under two paths: each is a duplicate in the engine's eyes. The canonical tag in the page's head (link rel=canonical) names the version to keep. Without it the engine picks one itself, not always the one you want.
The canonical should point to a page that exists, returns 200 and is not itself redirected or blocked. Filtered or paginated listings usually point to their unfiltered version; translated pages point to themselves and declare their siblings with hreflang.