Node.js is an open-source, back-end JavaScript runtime environment. Essentially, it allows developers to execute JavaScript code outside of a web browser, making it possible to build server-side applications with JavaScript. This is a big deal because it lets developers use the same language for both front-end and back-end development. Node.js is known for its efficiency in handling many simultaneous connections, making it suitable for real-time applications like chat servers and online games. It relies on an event-driven, non-blocking I/O model, which makes it lightweight and efficient.