Markdown is text formatting for people who do not want to fight a text editor. Instead of clicking buttons, you type characters: # Heading becomes a heading, **bold** becomes bold, and a line starting with - becomes a bullet. A file written this way is saved as .md, which is why you see that extension everywhere.
The point is that the file stays plain text: readable by any program, on any computer, forever. No layouts to break, nothing proprietary.
Why you keep hearing it
Because Markdown quietly became the native language of the AI era. Chat assistants write their answers in it. Notes apps like Notion and Obsidian run on it. And AI tools read their instructions from Markdown files: every project on GitHub has a README.md, and tools like Claude Code look for files like CLAUDE.md to learn how a project works. When someone says "put it in a Markdown file", they mean: write it as plain structured text the AI can read.
What it means for you
You already half-know it: if you have ever typed **text** or a # heading in a chat, that was Markdown. It is a ten-minute skill, and it makes your briefs, notes and instructions instantly usable by both humans and AI tools.