Amazon RDS: Multi-AZ vs Read Replicas.

In Simple Words:

• Multi-AZ: Synchronous standby replica in a second AZ. Dedicated to High Availability & Disaster Recovery; standby cannot accept read queries; automatic failover in < 60 seconds.
• Read Replica: Asynchronous replica dedicated to Read Scaling. Can be queried directly to offload read traffic; can be promoted to a standalone primary database.

resource "aws_db_instance" "replica" {
  identifier          = "postgres-read-replica"
  replicate_source_db = aws_db_instance.primary.identifier
  instance_class      = "db.r6g.large"
  publicly_accessible = false
}