Canonical URL
A canonical URL is the address a site declares to be the authoritative one for a piece of content, where the same thing is reachable at several addresses.
Stores generate duplicates constantly: the same product under two collections, with tracking parameters, with a currency or language variant. Without a declared canonical, each is a separate thing.
For agent readiness the practical consequence is in the feed: an item pointing at a non-canonical URL sends a shopper somewhere that may not be the page you maintain, and splits whatever signal the product has accumulated.
Why it matters to a store
A product represented at four addresses is four weaker records rather than one strong one, and the one an agent picked may not be the one whose price you keep current.