MERN Stack is an open-source JavaScript stack designed to simplify application development and deployment. 'MERN' stands for the stack's four technologies: MongoDB, Express.js, React, and Node.js. MongoDB is a cross-platform document-oriented database that supports ad-hoc queries, indexing, replication, and load balancing. Express is a Node.js back-end web application framework that may be used to create web apps and APIs. React is a front-end library that is used to create user interfaces. Finally, Node.js is a JavaScript runtime environment that enables programmers to run code outside of a web browser. MERN stack should not be confused with MEAN web dev stack, which utilises Angular.js as the JavaScript library instead of React.js.