When I started with Claude Code, I kept seeing files that ended in .md. CLAUDE.md. README.md. I didn't know what the letters meant.
They stand for Markdown. Markdown is plain text with a few symbols that add structure.
The handful of symbols
| What you type | What it looks like |
|---|---|
# A big heading | A big heading |
## A smaller heading | A smaller heading |
**bold words** | bold words |
*italic words* | italic words |
- a bullet | • a bullet |
1. a numbered step | 1. a numbered step |
That's enough to get started. You can open a Markdown file in any text editor and read it as is. Apps that understand Markdown turn the symbols into real formatting.
If you've ever seen an AI reply full of # and ** before it finished formatting, you were looking at MarkdownThe first time I opened a raw Markdown file, I thought my keyboard had gone haywire. Or the AI was having a meltdown and coughing up asterisks..
Why it matters if you use AI
AI writes in it. ChatGPT and Claude usually format their replies in Markdown.
AI reads it easily. The headings make it clear how a document is organized. Claude Code's instruction files, like CLAUDE.md, are Markdown too.
It's how I hand off work. When a Claude Code session gets too long, I have it save a summary as a .md file. I ask the fresh session to read that file, so it starts with what's done, what's left, and what we decided. Why sessions get too long is the next AI 101 post.
Even this blog postTechnically an .mdx file: Markdown plus a few extras for the website. is a Markdown file. When the site is built, its code turns each # into a heading and each ** into bold, then adds the website's fonts, colors, and layout. That's how a plain text file becomes the page you're reading.
Markdown is plain text that both people and AI can read.