The Markdown name plays on the “markup” part of the HTML acronym.

From the Markdown homepage

Markdown is a text-to-HTML conversion tool for web writers. Markdown allows you to write using an easy-to-read, easy-to-write plain text format, then convert it to structurally valid XHTML (or HTML).

You need a markdown converter to turn markdown into HTML. Many IDEs, static site generators and websites (like GitHub) support writing in markdown. There are other stands too like Asciidoc and RST.

Learn more on the Markdown guide site.

The Markdown Guide is a free and open-source reference guide that explains how to use Markdown, the simple and easy-to-use markup language you can use to format virtually any document.

Or read more on Wikipedia.

Markdown is a lightweight markup language for creating formatted text using a plain-text editor.

I also have Markdown cheatsheets if you need to brush up on the syntax or want to learn something new.