hreflang is an HTML attribute that declares the language versions of a page: this URL in English, that twin in French. It lets search engines send each visitor to the right version and prevents translations from competing against each other as duplicate content. Every page must reference all its twins, including itself.
Broken hreflang is one of the most common causes of a bilingual site underperforming in one language — our guide to multilingual website SEO walks through the correct setup.