In this video of the MongoDB series, the speaker introduces MongoDB as a popular NoSQL database that offers flexibility with its document-oriented structure. MongoDB was released in 2009, replacing traditional rows with documents, allowing for embedded documents and arrays. Its schema-free nature enables easier handling of evolving data models and eliminates the need for fixed keys. MongoDB's widespread use can be attributed to its flexibility, native aggregation capabilities, and schema-free model. The database is known for its general-purpose nature, flexible schema design, scalability through sharding, and built-in aggregation framework.