MERN Stack is a full-stack web development stack consisting of MongoDB, Express, React, and Node.js. It is a popular stack for building modern web applications as it provides a way to build both the front-end and back-end of a web application with JavaScript. MongoDB is a NoSQL database, Express is a web framework for Node.js, React is a popular JavaScript library for building user interfaces, and Node.js is a server-side platform for building fast and scalable web applications.