Markdown is the language used to format the plain text files in your Git-enabled repository. Its also called a markup language, like HTML. Both languages add markers to plain text to style and organize the text of a document. In HTML:
<h1> Heading 1 </h1>
# Heading 1
Whereas you use HTML and CSS with WordPress, you use Markdown with Git and GitHub. Markdown has fewer options for marking text than HTML. It was designed to be human-readable, meaning easy to write and edit.