By default, AWS service instances are created in isolation and can't access other services without your explicit permission.
We need to give our Elastic Compute Cloud (EC2) instance access to the Relational database service (RDS) we just created. To do so, we will edit the inbound rules of our database to include the security group that we set for EC2. This will allow access for every item in that security group but currently we only have our EC2 instance.