HTML stands for HyperText Markup Language and it's a standard language to create website contents. It was first released in 1993 and has ever since dominated the World Wide Web. Most websites to this date have code based on HTML to display their content. Some basics were already covered in Chapter 11, Working with the Qlik Dev Hub.
HTML is a semantic language which allows you to organize and denote the structure of a website using tags such as <header>, <body>, and <footer> or more structural elements such as <img>, <section>, or <p>. It's very rich in its offering and in combination with CSS and JavaScript forms the three core cornerstones of the World Wide Web.