Hreflang
Hreflang is an annotation telling search engines which language or regional version of a page to serve to which audience. It prevents your US page outranking your UK page for UK searchers, and stops translated versions being treated as duplicates.
Why it matters for your rankings
Hreflang is the most error-prone technical implementation in SEO. It requires every version to reference every other version, including itself, and a single missing return tag invalidates the whole cluster.
The consequences are commercial rather than abstract. Without working hreflang, a British customer sees dollar pricing and a US shipping policy, and bounces. With broken hreflang, Google falls back to guessing, which usually favours whichever version has the most links, so smaller markets get the wrong page permanently. The other frequent mistake is using it to solve a problem it does not address: hreflang manages which version is shown, not which ranks. It is not a substitute for genuinely localised content.
How to check it on your site
Check return tags
Every page in a language cluster must link to all others and to itself. Screaming Frog's Hreflang tab flags missing confirmation links.
Validate language and region codes
Use ISO 639-1 for language and ISO 3166-1 Alpha 2 for region. 'en-UK' is invalid, 'en-GB' is correct.
Add an x-default
Specify an x-default version as the fallback for audiences whose language and region match none of the versions you have listed.
Confirm URLs are absolute and indexable
Hreflang targets must be canonical, indexable, absolute URLs. Pointing at a noindexed or redirected page breaks the cluster.