Multi-AZ RDS, not to be confused with read replicas, allows you to create a standby instances for your RDS, this has neither read nor write capabilities. It is synchronized atomically with the primary DB in fact the original RDS must be able to forward the information to the standby RDS before it will save it to ensure it is updated properly. It is a high availability / disaster recovery option, when the main RDS fails the replica is under the same DNS name and it will automatically fail over. I should note as well you can also utilize read replicas for a multi-AZ setup alongside your standby RDS, promoting read replicas to the primary RDS in the event of failure, or even creating standby instances of the read replica itself. It is important to note the standby RDS is only for failover purposes. It will not do much good if you want to lighten the load on your primary RDS as it doesn’t allow read / write. That’s where read replicas come in.