It stands for Hypertext Markup Language but that is not important.
When a very clever person invented the World Wide Web he needed a way to create the pages (and the links between them).
A markup language is where you add stuff to text to structure it in some way. HTML is a language to structure text documents, describe their contents and create the links between them.
Markup languages add tags to text to structure them. HTML tags identify the parts of the page. For example paragraphs, lists or tables. These tags tell your Web browser what each part is so the browser can handle the text properly.