Node.js is a JavaScript runtime built on Chrome's JavaScript engine for building fast and scalable network applications. It uses non-blocking I/O and event-driven architecture, making it suitable for real-time applications with many simultaneous connections. The document provides an example of an HTTP server in Node.js and discusses key aspects of the platform like the event loop, modules, and package management with npm. It also introduces Express.js as a popular web framework built on Node.js that simplifies building web servers.