Hreflang is an annotation that tells search engines which pages are language or regional versions of the same content. That way Google can show a Dutch searcher the Dutch page and a British searcher the English one. Google supports three equivalent methods: <link> elements in the HTML, an HTTP header or entries in the XML sitemap.
Example
A site with a Dutch and an English version puts the same set of lines on both pages:
<link rel="alternate" hreflang="nl" href="https://www.example.com/diensten">
<link rel="alternate" hreflang="en" href="https://www.example.com/en/services">
<link rel="alternate" hreflang="x-default" href="https://www.example.com/diensten">The value is a language code in ISO 639-1 format, optionally followed by a region code in ISO 3166-1 Alpha 2 format, such as nl-BE. A region code on its own is not valid. With x-default you name a default page for searchers whom no version fits.
Common mistake
References that only go one way. According to Google, each language version must list itself and all the other versions. If the English page does not point back to the Dutch one, Google may ignore the annotation. Addresses without https:// also often go wrong.
How do you check it?
Search the page source of each version for hreflang and compare the sets. Atlas, the research engine developed by Outseen, checks language references between the fetched pages. More checks are in the technical SEO checklist.