Tags that you put on links to different language versions of your site.
If your site has 3 languages, you would put links on your pages to your other languages with hreflang tags
If you have hreflang tags and google has properly indexed them then localized versions of your site will appear in serps.
I.E.: a spanish user searches for something in english, something that the english version of your site would rank for. Instead of giving them the english version, google swaps that result with the localised version.