A replica set in MongoDB is a group of MongoDB instances that maintain the same data set. This means that each instance of the replica set contains the same data, and any changes made to the data on one instance will be automatically replicated to the other instances. For example, if you write a document to one instance, that document will be automatically replicated to the other instances in the replica set. Replica sets also provide redundancy and high availability, as they can continue to serve data even if one of the instances fails.