MongoDB is the "M" in MERN stack, which stands for MongoDB, Express, React, and Node.js. MongoDB is a NoSQL document-oriented database that is used to store data in JSON-like documents. It is highly scalable, flexible, and can handle large amounts of unstructured data. MongoDB's document-based structure allows for easy and fast querying of data, making it an ideal choice for modern web applications. MongoDB also supports advanced features such as sharding, replication, and aggregation, making it a powerful database management system for MERN stack developers. Its open-source nature and active community ensure regular updates and a high level of support for developers.