Markdown is a simplest-possible (well...) system for marking up (get it?) text for processing, formatting, or publication. The set of possibilities in markdown are roughly equivalent to that of HTML markup, but it is much simpler, less verbose, and therefore easy to read. Markdown files are just plain text files with minimal additions.
The simple 'markup' used in editing this wiki is not full markdown, but an older system (MediaWiki markup] which is similar in spirit but less standard and less comprehensive.
Publishing and conversion apps like Pandoc and Manifold use markdown, as does the open-source software repositories GitLab and GitHub.
A very simple example of a markdown document:
# First-Level Heading. This is a paragraph. This is another paragraph. ## Second-Level Heading Another paragraph. > Block quoted paragraph. - list item - list item
A document marked up like this can be converted directly to HTML (or to other formats) by a host of conversion software (e.g. Pandoc)
For a much fuller explanation of markdown, see:
- John Gruber's original markdown specification
- CommonMark, a newer, more robust version of the standard
- Krycho, Chris. "Academic Markdown and Citations." July 26, 2015.
- Selisker, Scott. "A Plain Text Workflow for Academic Writing with Atom." January 23, 2019.