What is the HeavyUsage charge on AWS EC2?

You have committed to pay for this type of server for three years with a no upfront reservation. This line item is the monthly fee for the server you have reserved.

AWS payment is typically in arrears as it's usage base. Given this is taken in advance I guess it's because they know in advance what the fee will be.

I was under the impression that AWS discontinued the Light / Medium / Heavy Utilization Reserved instances. For their description look here: EC2 Reserved Instance Options, however note that the Light and Medium options are no longer available.

Essentially the heavy utilization means that your instance is running 24/7. It doesn't matter how busy the instance was, only that it was on.

These options were quite confusing and I believe they're no longer offered.

Hope that helps :)