MongoDB is a popular, open-source, No SQL database program. It’s designed to handle large volumes of data and is known for its flexibility, scalability, and performance. MongoDB uses a document-oriented data model, storing data in flexible, JSON-like documents. These documents can vary in structure, making MongoDB suitable for managing unstructured or semi-structured data.

Key features and concepts of MongoDB include:

1. Document-Oriented: Data is stored in flexible, JSON-like documents, making it easy to model and store data in a way that matches your application’s needs.

2. Scalability: MongoDB is designed to scale out horizontally by adding more servers to distribute the load. It allows for shading, which divides data across multiple servers to improve performance as data grows.

3. High Performance: With its indexing capabilities, efficient querying, and support for in-memory computing, MongoDB can deliver high performance for read and write operations.

4. Replication and High Availability: MongoDB provides replication, allowing data to be mirrored across multiple servers, ensuring high availability and fault tolerance.

5. Querying: MongoDB offers a powerful query language that supports complex queries, indexing, aggregation, and more.

6. Ad Hoc Queries: Developers can perform ad hoc queries on data, making it flexible for changing requirements.

7. Geospatial Queries: It has support for geospatial queries, which is useful for location-based applications.