What is Canonical URL?
A canonical URL is the single official address for a page, declared so search engines know which version to index when the same content is reachable through several URLs.
Ecommerce sites generate duplicates constantly — the same product reachable through multiple categories, or with filter and tracking parameters attached. Without a canonical, those variants compete with each other and split the page's ranking signals.
A well-built storefront sets this automatically for every product and category page.