SEAN.JS is a web application boilerplate, which means you should start changing everything :-)
This sample application tracks users and articles.
Sequelize is an ORM for relational databases such as PostgreSQL. Sequelize's great manual is the place to get started with it.
Express is an app server. Check out The Express Guide 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.
As this boilerplate is based on MEAN.JS, you can get more information on their site and documentation