Markdown is a lightweight markup language which uses plain text to describe formatting information, such as # Heading, 1. numbered list, **bold**, etc. This add-on identifies markdown documents by the extension in the URL (.md, .markdown, etc.). When you navigate to a markdown document, if the content is plain text, this add-on formats it into HTML (with headings, bold text, etc.) as described in the document and displays it directly in your browser.