HTML5 is a markup language for structuring and presenting content of your pages
HTML5 - the markup language for structuring and presenting content for the World Wide Web, as well as, the main technology used on the Internet. This is the fifth version of the HTML standard. Its main aim is to improve the language that supports the latest multimedia applications, while retaining the ease of reading the code for the person and the clarity of performance for computers and devices.
In other words:
HTML - a language that allows the presentation of information (eg, research) on the Internet. What you see when you view the page on the Internet, is your browser's interpretation of the HTML code. All the elements are defined by so-called tags. The fifth edition of this markup language can easily integrate a significant number of multimedia elements, such as video, SVG-graphics, etc., onto the pages your site.
What is it used for?
HTML is used to build web pages in the browser. HTML5 is useful for the proper structuring of information, in accordance with the modern principles of web development, as well as, to accommodate the multimedia elements on the site.