Labels around content
In HTML, <h1> marks a heading. This is markup: extra symbols that describe what text means. Browsers use those labels to build a page. CSS usually controls how it looks.
Markdown uses lighter marks. For example, # Heading marks a heading and **important** marks bold text. People can read the plain text even before it is displayed as a formatted page. Documentation sites and notes often turn Markdown into HTML. Markdown varies slightly between tools, so a feature that works in one editor may not work in another.