MongoDB is a NoSQL database, which allows for the storage and retrieval of data in a JSON-like format. It is a popular choice for web applications because it is flexible and can handle large amounts of unstructured data.
ExpressJS is a web application framework built on top of Node.js. It simplifies the process of handling HTTP requests and responses, and it provides a robust set of features for web and mobile applications.
Overall, a MEAN stack application is a web application that uses MongoDB, ExpressJS, AngularJS, and Node.js to handle the database, the server-side logic, the client-side logic, and the server runtime respectively. It allows developers to use one language throughout the entire application stack, and makes it easier to maintain and scale.