A replica set is a group of MongoDB instances that store the same set of data. Replicas are basically used in production to ensure a high availability of data.

Redundancy and data availability: because of replication, we have redundant data across the MongoDB instances. We are using replication to provide a high availability of data to the application. If one instance of MongoDB is unavailable, we can serve data from another instance. Replication also increases the read capacity of applications as reading operations can be sent to different servers and retrieve data faster. By maintaining data on different servers, we can increase the locality of data and increase the availability of data for distributed applications. We can use the replica copy for backup, reporting, as well as disaster recovery.