hreflang

hreflang is markup telling search engines which language and country each version of a page is meant for.

In a little more detail

It only matters if you publish in more than one language. When it is wrong, the usual symptom is the wrong country's page showing to the wrong people.

Related terms

Canonical: A canonical tag tells a search engine which version of a page is the real one when several addresses show the same content.
Index: The index is the searchable copy of the web a search engine keeps. If a page is not in it, that page cannot appear in results.

See it on your own site

The scan reads your site the way AI search does and reports where you stand on this and sixteen other checks.