Markdown is plain text formatting design for online content writer, which nowadays getting more hyped. In essential markdown it's a way formatting text when writing for online content without knowing HTML / CSS code. It is an alternative to WYSIWYG (what you see is what you get) editors, which use rich text that later gets converted to proper HTML.
This article offers a sample of basic Markdown syntax that can be used in Hugo content files, also it shows whether basic HTML elements are decorated with CSS in a Hugo theme.