For EC2, VMware Aria Cost (CloudHealth) calculates the RIs as a “pool” of hours that you have committed to spending each month.
 
This pool is displayed as an Unused RI cost on the first day of a month. With each passing hour, that pool is consumed as RIs are applied to instances. Therefore, Unused RI should decrease with each passing day as the equivalent EC2 Compute cost goes up.
 
At the end of the month, any remaining Unused RI cost for EC2 under Committed Charges is the total number of hours that were not applied to instances.
 
In summary, it is the amount you committed to pay Amazon through EC2 RI's and it burns down over the course of the month as EC2 usage receiving the RI benefit increases.
 
NOTE: Committed Charges does not include Unused RI for other service types as AWS does not burn down the usage in the same way.