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:
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.
Learn how to take a handful of static HTML files and convert them into templated files that will help you minimize errors and work more efficiently.
It takes a lot to bring an idea to life on the web, even for the simplest of sites. Follow this guide for a detailed look at moving from concept to a website deployed to your domain.