A canonical URL is the address you tell search engines to treat as the main version of a page when the same content can be reached at several addresses. It is set with a link rel="canonical" tag.
Duplicates appear without anyone meaning to create them: with and without www, with tracking parameters, with sort and filter options. The canonical tag gathers them under one address.
It is a strong hint, not a command. Search engines can pick a different canonical if your internal links and sitemap point elsewhere.
Why it matters for a launch
A site without canonicals splits its own ranking signals across duplicate addresses. It is a one-line fix that is easy to forget on a new site.
Read more: Why a new website isn't on Google yet