This workflow terminates all the instances which are idle from the past 7 days and are launched before 7 days. Instance is identified as idle if its CPU Utilisation is less than 2% and Network In/Out is less than 5MB. You can also give other configurations for this workflow.
This workflows send a report of under-utilised(default 40%) EC2 instances. You can save cost by downsizing under-utilised EC2 instances.