The software world
.gitignore
A file in your project listing what git should never track —
.env files, credentials, build output, huge generated folders. Anything matching a line in it stays on your machine and out of every commit. Why it matters
Part of Version control (git) → It's the one piece of git that exists mainly to stop a disaster. A secret that reaches a commit is in the history permanently — deleting it in the next commit doesn't remove it from the old one, and once pushed you must assume it's compromised. .gitignore is what keeps it from getting there. You don't have to write one from memory: ask Claude "is anything here that shouldn't go into git?" before your first commit in a new project.
see also