Markdown is a lightweight markup language created by John Gruber in 2004. It adds a handful of symbols to plain text to express structure like headings, lists and links. Its key strength is that the source itself is easy to read, so you can grasp the content even before it is converted.
For example, a line starting with # becomes a heading, and a line starting with - becomes a list. A document written this way is converted to HTML and displays consistently on web pages, blogs and documentation sites.
Today countless tools adopt Markdown as a standard: GitHub READMEs, developer docs, note apps like Notion and Obsidian, and static site generators. Learn it once and you can use it across many platforms.
Paste some Markdown into the AG MD converter to see exactly what HTML it produces. When a rule is unclear, check the per-item examples in the cheat sheet.