In more detail
The same content often ends up on several URLs: with and without www, with tracking parameters, in print versions, or through product filters. Search engines then have to choose one to show. Google calls this process canonicalisation.
A canonical tag, placed in the page's <head> as <link rel="canonical" href="https://example.com/page">, states your preference. Google's guide to consolidating duplicate URLs lists several ways to signal it, from strongest to weakest: 301 redirects, rel="canonical" annotations, and inclusion in a sitemap. Google may still choose a different URL if other signals disagree, for example if internal links all point elsewhere.
Common mistakes include canonical tags pointing to pages that redirect or return errors, every page on a site pointing to the home page, and canonical tags that contradict hreflang annotations on international sites. Mixing a canonical tag with noindex on the same page sends conflicting signals.
Why it matters when hiring an agency
Canonical problems are among the most common findings in a technical SEO audit, especially on ecommerce sites with filtered category pages. Fixing them can concentrate ranking signals on the pages you want to rank, and stop search engines wasting time on duplicates.
Ask an agency how it will check which URL Google has actually chosen. The URL Inspection tool in Search Console shows both the user-declared and the Google-selected canonical. Our Google Index Checker also reports the canonical a page declares, so you can spot obvious errors before and after changes.