The term MongoDB is established from the word Humongous because it can store a large amount of data. Most importantly, MongoDB is a database server that basically allows you to run the different databases on it.
MongoDB is a non-relational DBMS which means that data is not stored in it using the tables. However, you have a schemaless structure on which MongoDB works and this is the flexibility that MongoDB gives you.
A MongoDB is a database solution that stores data inside collections. Below, the diagram is just a pictorial representation of how we have different collections stored in MongoDB.
Inside, every collection the data is stored in a Key-values format. That key-value objects are nothing but JSON objects.
Unlike SQL database, MongoDB follows horizontal scaling. This is the most important feature because of which MongoDb is in high demand now a days.