Node.JS is like getting rid of the browser-prison for JavaScript. You can build any application you can come up with, thanks to JavaScript on Node.JS. While most people use it for building a specific webserver, there are more usages, like mobile apps (Cordova), which can be easily created in HTML/CSS/JavaScript thanks to Node.JS, you can build desktop apps, which run directly on Node.JS (Electron). And there are even some guys building an operating system on top of Node.JS (NodeOS).
I'd say, it is very comparable to Python, minus some speed plus the merits of JavaScript. If you are a Web Dev, Node.JS is a must-know :)