Node.js is a JavaScript runtime that runs on the server side. It allows developers to create scalable, high-performance applications using event-driven, non-blocking I/O. Users can take quizzes to test their understanding of Node.js concepts such as asynchronous programming, Express.js routing, REST APIs, working with databases like MongoDB, and real-time applications using WebSockets. Completing these challenges helps users gain practical skills for server-side development with JavaScript.