The MEAN stack is a full-stack JavaScript solution that uses MongoDB, Express, AngularJS, and Node.js. It allows developers to build fast, robust, and maintainable web applications using a uniform language (JavaScript) and data format (JSON) throughout the stack. MongoDB is a NoSQL database that stores data as JSON documents and provides features like indexing, replication, and rich queries. Express is a flexible Node.js web application framework. Angular is a JavaScript framework for building web apps and provides features like data binding, directives, and modularity. Node.js is an event-driven, non-blocking web server that makes the MEAN stack lightweight and efficient for real-time applications.