MongoDB is an open source, cross-platform, and the most popular NoSQL database program. Each database has collections which in turn has documents. The data stored is in the form of JSON style documents (rows).
Schemaless: Documents don't need to have a schema defined beforehand. Instead, the creation of the field happens on the fly.
Fast Access: It uses internal memory for storing the (windowed) working set, enabling faster access to data.
Replication: MongoDB can provide high availability with replica sets. A replica set consists of two or more mongo DB instances.
No complex joins : instead of joins, it supports embedding or referencing to access related data
No need for complex ORM (mapping from relational database tables to application objects )
Language support: It has official drivers for major programming languages and development environments.
JSON based structure allows to store complex nested schemas. It plays well with NodeJS , which also understand json natively
One can learn Mongo Database from Mongo Database University(https://university.mongodb.com/). They offer the free courses with certification after completing it successfully.