In more detail
Hreflang annotations take the form <link rel="alternate" hreflang="en-au" href="https://example.com/au/">. Each version of a page lists all its alternates, including itself, using ISO language codes and, optionally, region codes. An x-default value can name the fallback page for users who don't match any listed version. Google's guide to localised versions of your pages explains three ways to supply them: HTML tags in the page head, HTTP headers, or an XML sitemap.
The rules are strict. Annotations must be reciprocal: if page A lists page B, page B must list page A. Codes must be valid (for example, "en-uk" is wrong; the correct region code is "gb"). Each URL should be the version Google can index, not a page that redirects or carries a canonical tag pointing elsewhere.
Hreflang doesn't stop duplicate-content consolidation on its own and isn't a ranking boost. It helps Google choose which version to show. Google's guide to managing multi-regional and multilingual sites also covers URL structures, such as country domains, subdomains and subfolders.
Why it matters when hiring an agency
Hreflang errors are common and often invisible until the wrong country version starts ranking. If you operate in several countries or languages, ask agencies about their international experience, one of the questions Google itself suggests asking.
Ask how they'll audit hreflang at scale, how they'll keep it correct as pages are added or removed, and how it interacts with your CMS. For large sites, managing hreflang in sitemaps is often more reliable than in page templates. Expect international work to cost more, because each market adds research, content and technical checks.