HTML is the foundation of web pages. It stands for hypertext markup language.
As a user of the web, when you arrive on any page — like this one, perhaps — the process of getting there looks something like this:
- Open a web browser application on your device.
- Either click a link to this page or type the URL directly into the address bar.
- The web server running this website receives the request from you and your browser, and sends the browser back some HTML code.
- The browser downloads that HTML code, parses it, and displays the result on your screen.
That's why this page displays the content you're reading now. HTML is the language used to represent the various elements that make up this page.
There are so many resources on HTML out there today that I'm not going to try to recreate them beyond this quick definition. If you're new to the web and/or web development, I'd suggest going through a free tutorial like this one from freeCodeCamp or this one from Codeacademy.