What is HTML?
In one sentence: The language web pages are written in: headings, paragraphs, links and so on.
A little more
HTML (HyperText Markup Language) is the text a web server sends to describe a page: this is a heading, this is a paragraph, here's a picture, this word links somewhere else. The browser reads it and draws the page.
HTML says what things are. CSS (style sheets) says how they look, and JavaScript makes them do things.
It's a bit like… The labels on a recipe: Title, Ingredients, Method. They tell the cook what each part is so it can be laid out properly.