Definition
hreflang is a <link> tag declaring the language and optional regional variants of a page: <link rel='alternate' hreflang='es' href='https://example.com/es/page' />. Search and AI engines use this to serve the right locale variant to each user. Without hreflang, engines may serve the English version to Spanish users (or vice versa), and may treat translations as duplicate content.
An ecommerce site with /en, /es, /fr versions. Each page declares all 3 hreflang variants in <head> so the right one surfaces per locale.
How to optimize
Add hreflang for every locale variant on every page. Use ISO 639-1 language codes + ISO 3166-1 region codes. Add x-default for the fallback locale.