A `.env` file lists shell-style `KEY=value` lines that get loaded as environment variables by tools that support dotenv (Node's dotenv package, Docker Compose, Laravel, Rails, Vercel). Originated as a Twelve-Factor App convention (2011) and now ubiquitous in web app deployment. Lines starting with `#` are comments. Values with spaces or special chars need quoting.
How to open
Any text editor. macOS Finder hides files starting with `.` by default — Cmd+Shift+. to toggle visibility. Most IDEs and code editors syntax-highlight .env files.