Markdown is a markup language created by John Gruber (with contributions from Aaron Swartz) in 2004. There is no "official" specification other than Gruber's original syntax release, but there are formalised variants of it. Two notable specifications are CommonMark and its superset GitHub Flavored Markdown. The Ochrs generator used for this site uses Python-Markdown which is "almost completely compliant" with Gruber's syntax, plus a number of extensions to add features from CommonMark (such as fenced code blocks).
Background
Computers have been used for typesetting for a long time. Roff (runoff) predates Unix and was used for this since the 1960s. This was superseded by Nroff (new roff), and later troff, which is the basis for Unix man pages. TeX (and LaTeX) are common now in academia for printed documents. HTML is designed for use on a screen rather than in print, based around the idea of page markup.
Markdown is designed to simply the process of writing text markup, and aides generally in the creation of HTML. HTML places the formatting instructions within the document, but it is very cumbersome to write. Markdown moves tags and tree structure into simpler character-based syntax.
Markdown is widely used in popular programs, such as Discord, Reddit, GitHub, and Obsidian.