Node.js is a runtime environment for server-side applications.
Node.js enables writing backend services using Java Script.
Node.js uses the same V8 Java Script engine that is used in the Chrome browser to optimize the execution of Java Script code.
Node.js is event driven in nature, and built towards the creation of services that require a lot of messaging and networking. This makes it suitable for signaling.
Node.js is very popular in the development of many WebRTC related services.