Node.js is a popular open-source JavaScript runtime environment for building server-side applications. It is built on top of the Chrome V8 JavaScript engine and allows developers to write server-side code in JavaScript.

One of the key benefits of using Node.js is its ability to handle a large number of simultaneous connections, making it well-suited for building scalable and performant applications. Node.js also has a rich ecosystem of packages and modules available through npm, the largest package manager for JavaScript, which makes it easy to add functionality to your applications.

Node.js is also lightweight and fast, which makes it ideal for building high-performance and low-latency applications. This, combined with its event-driven, non-blocking I/O model, makes Node.js a great choice for building real-time applications, such as chat applications and online games.

Another benefit of Node.js is its large and growing community of developers, with many available resources such as tutorials, forums, and packages to help you get started quickly. The runtime environment is also highly customizable, with a range of plugins and configuration options available to meet the specific needs of different projects.

Whether you're a seasoned web developer or just starting out, Node.js is a great choice for building server-side applications. With its performance, scalability, and ease of use, it's no wonder that Node.js is one of the most popular runtime environments for building server-side applications today.