MEAN stands for MongoDB, Express, Angular and Node.
MongoDB is a noSQL database that uses JSON-style documents for data representation, making it ideal for use with Javascript.
ExpressJS is a web application framework written in JavaScript that is used with Node.js to help boost it up.
AngularJS is JavaScript framework by Google that allows for complex client side applications and includes two-way data binding.
Node.js is a JavaScript framework that is concurrent and built on V8’, for easily building fast and scalable web applications.
Combining all 4 of these allows for full-stack JavaScript development.