A MERN stack web developer specializes in using MongoDB, Express.js, React.js, and Node.js technologies for web development. They design and implement user-facing features using React.js and build robust, scalable backend services in Node.js. They are responsible for creating database schemas with MongoDB and using Express.js as the web application framework. They also ensure the responsiveness and efficiency of applications, troubleshoot and debug, and maintain code quality. They are involved in the entire web development process from conception to deployment, often working with other team members like UI/UX designers and quality assurance testers.