Possible Savings

What does the "Available Savings" mean?

It is the total saving that is currently available from un-used and underutilized AWS Resources.
By default an instance is categorized as underutilized if the cpu utilization and Network I/O is less than 5%. This is configurable in the rules page.
Click on the EC2 underutilized section on possible savings page.
These are savings that you will get from EC2 instances stopped for a while wich can be terminated.