Markdown is a markup language that's basically simplified HTML. You just focus on writing without the other stuff and the rest follows.
There are different flavors of Markdown, such as kramdown. But, it's still Markdown. I have used Markdown to create some documentation or to communicate on a project's status in GitHub. When I did that, it felt pretty freeing to focus on what I wanted to say. Creating Markdown files in Atom for some documentation was pretty fun too.
I'm even using Markdown to create an early draft for my next book.
If I had my druthers, I would create all technical documentation in Markdown. I wish I could just do all my technical writing magic in this markup language.
Anyway, here are some places to get a good understanding on Markdown. (Thank you, John Gruber for creating Markdown. I appreciate it.)
I hope this aids you in your journey. Keep on writing.