MongoDB is very flexible and it is a blessing and a curse. It will store any data you provide. By default there is no validition you setup validation rules on database or use ODM like Mongoose if you're using NodeJs to help with validation in the application layer.
NoSQL is a term used to describe a class of non-relational database management systems that are designed for large-scale, distributed data storage and retrieval. Unlike traditional relational databases, which use a fixed schema and use SQL for data access, NoSQL databases are schema-less and use more flexible query languages, such as MapReduce, to allow for more agile development and real-time data processing. NoSQL databases are often used for big data and real-time web applications.