In Atlas, a cluster means both the managed service that sets up the architecture around your database to support your needs, such as scalability, and the generic infrastructure term for a group of servers. When you create the default Free cluster, Atlas may name it Cluster0, unless you choose another name. A Free cluster is a sandbox replica set. In MongoDB terms, a replica set has members, or nodes, and one member is elected primary while the others are secondary members. In the Atlas UI, go to Database > Clusters > View Monitoring from the cluster card to see the replica set members and their current roles.