Markdown is a markup language designed for blogging. It's like HTML, with simpler syntax, designed for blogging.
Markdown syntax from Commonmark . Commonmark is one popular flavour of MD. There are many others.
As a language it's actually more complex than HTML, because it includes HTML.
But generally, usage of Markdown means writing documents that are closer to plain text, with small quantities of markup symbols.
Great if you're used to HTML.
I'm going to use it for formatting text in Spatial notes.
It has problems tho!