Markdown is a plain-text formatting syntax. It enables you write documents in plain text, readable by others in plain text, and optionally rendered into nicely formatted PDF, HTML, DOCX etc. It’s used widely in software documentation, particularly open-source, because it enables richer formatting than plain-text alone, but without constraining authors or readers to a given software platform. Platforms such as github natively support Markdown rendering - so you write your README etc in markdown, and when viewed on github it is automagically rendered - without you needing to actually do anything.