The MEAN Stack is a collection of JavaScript-based technologies used to develop web applications, comprising MongoDB, Express.js, Angular, and Node.js. This stack enables developers to use a single language, JavaScript, across the entire application, from the client-side interface to the server-side logic and database management. MongoDB serves as the NoSQL database, Express.js as the web application framework, Angular as the front-end framework, and Node.js as the server runtime environment. The MEAN stack is known for its efficiency, scalability, and flexibility, making it a popular choice for building dynamic, full-stack web applications.