This metric is relevant to RDS databases running in a T2 level instance. The more vCPUs available to the instance, the higher the number of credits per hour should be.
If the balance of credits is consistently low, this indicates that the workload is using more CPU than the instance can support. If the value is consistently high, then the instance may be too large for the workload it is supporting. CPU Credit Balance is collected only at 5-minute intervals.