MEAN.JS is a web application boilerplate, which means you should start changing everything :-)
This sample application tracks users and articles.
MongoDB is a database. MongoDB's great manual is the place to get started with NoSQL and MongoDB.
Express is an app server. Check out The ExpressJS API reference for more information or StackOverflow for more info.
AngularJS is web app framework. Angular's website offers a lot. The Thinkster Popular Guide and Egghead Videos are great resources.
Node.js is a web server. 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: