Many markup and style sheet languages and protocols (HTML, CSS, XML, HTTP, and so on) use language tags in one way or another. Those language tags specify the language of the marked up or styled text.
There are two international standards for encoding languages: ISO 639-1 two-letter language codes and ISO 639-2 three-letter codes. When looking up the language tag to use on the web, neither should be used. The official registry of language tags that should be used instead of ISO codes is the IANA Language Subtag Registry.
Many websites have content in multiple languages targeting multiple countries. For the purposes of SEO, it is important to explicitly specify the target region and language codes for each page and link different page versions to each other.
Normally, each page only includes content in one primary language and targets a specific country or region.