Markdown is a lightweight markup language that allows you to create HTML using simple symbols. For example, doing *beer* creates an italic-styled HTML wrapper for the word. Markdown is a great way to accept user-generated content on a site without having to worry about the complexities of HTML.
Brackets architect, Glenn Ruehle, created an extension to help make using Markdown easier. Simply install his extension directly from the GitHub and when you open any .MD file, you will find a new panel that contains a live preview of your Markdown text. It’s incredibly helpful and I plan on using it whenever working on my ReadMe files for GitHub!