MEAN.JS is a web application boilerplate, which means you should start changing everything :-)
This sample application tracks users and articles.
MongoDB is a NoSQL database. MongoDB's great manual is the place to get started with NoSQL and MongoDB.
Express is a NodeJS server-side application web framework. Check out The ExpressJS API reference for more information.
AngularJS is client-side web application framework. The Thinkster Popular Guide and Egghead Videos are great resources.
Node.js is a JavaScript run-time, popular for being a web server platform. Node's website and this stackOverflow thread offer excellent starting points to get to grasps with node.
Once you're familiar with the foundation technology, check out the MEAN.JS Documentation: