It is a markup language, not a programming language, used to create web pages. Its name is an acronym for HyperText Markup Language.
The language is used to define the content of a web page: it determines what should be placed where (in cooperation with CSS) and what exactly it is, whether text, a quotation, an image, a video and so on.
This language is very similar to XML, which is derived from it. HTML has a defined structure that must be followed for the page to be displayed correctly by a web browser. If the rules are not followed, so-called invalid code results.
Use
HTML is the absolute foundation of all web pages. To create such pages, we can write all the tags by hand, or we can use so-called WYSIWYG editors, which make it possible to create pages even without knowing HTML.
The HTML 5 concept now makes it easy to embed videos, music and other media files in pages. This used to be very difficult and was usually solved with helper technologies such as JavaScript or Flash.