MERN stack developer refers to a software developer who specializes in using a specific set of technologies to build web applications. The term "MERN" stands for MongoDB, Express.js, React.js, and Node.js, which are the key technologies used in this stack.

Here's what each component of the MERN stack represents:

1. MongoDB: MongoDB is a popular NoSQL database management system that stores data in a flexible, JSON-like format. It is known for its scalability, flexibility, and ease of use.
2. Express.js: Express.js is a minimalist web application framework for Node.js. It simplifies the process of building robust web applications and APIs by providing a set of powerful features and middleware that helps developers manage routes, handle requests and responses, and interact with databases.
3. React.js: React.js is a JavaScript library for building user interfaces. Developed and maintained by Facebook, React.js allows developers to create reusable UI components and manage the state of the application efficiently. It is widely used for building interactive and dynamic user interfaces for single-page applications.
4. Node.js: Node.js is a server-side JavaScript runtime environment that allows developers to build scalable and high-performance network applications. It uses an event-driven, non-blocking I/O model, making it ideal for building data-intensive real-time applications.

MERN stack developers are proficient in working with these technologies and can build end-to-end web applications, handling both the frontend (user interface) and the backend (server-side logic and database interactions). This stack is popular because it allows developers to use JavaScript throughout the entire application, which can lead to increased productivity and code consistency.