Node.js is an open-source runtime environment that allows developers to execute JavaScript on the server side. It is event-driven and non-blocking, making it ideal for building real-time applications, such as chat apps, gaming servers, and live streaming platforms. Node.js has a rich ecosystem of libraries and modules available through npm (Node Package Manager), which speeds up development. Its asynchronous nature and scalability make it suitable for high-performance applications handling multiple concurrent connections. Node.js is a popular choice for backend development in modern web applications.