The hreflang attribute lives on <link rel="alternate"> tags in <head> (or in the sitemap, or in HTTP Link headers) and tells search engines that two URLs are language/region variants of the same content. It's how Google decides whether to show the French or German version of your product page to a French user in Switzerland.
Format: hreflang="en-US" (BCP 47 language + optional region). The reciprocal rule is non-negotiable: if page A declares B as its French version, B MUST declare A as its English version, otherwise Google ignores both annotations.
The x-default value (hreflang="x-default") marks the fallback URL when no language matches. Always include it -- Google uses it for users in countries you haven't translated for.