It's a file Claude Code automatically reads at the start of every session. Anything in it is basically part of every prompt, so you don't have to repeat yourself.
Create it with /init inside Claude Code: it scans your project and writes a first version. Then edit it yourself. Good things to put in:
- What the app is, in two sentences.
- Stack and important versions.
- Commands: how to run, test, build.
- Rules you keep repeating ("use the server client in app/", "never edit migrations that already ran").
Keep it short. Every line costs context in every session, and a long file gets followed less.
There's also ~/.claude/CLAUDE.md for personal preferences across all projects.