Fee-free savings credits offset any fees that would normally be charged using Card Deals.
For example, let's say we've activated a deal for FedEx on your Amex. You're at FedEx, you open MaxRewards and you see that you should be using your Amex because you'll save 20%. You spend $100 and get $20 in statement credit. You're happy because without MaxRewards, you wouldn't have even known about the FedEx deal.
Normally, we'd charge you 20% of your savings or $4. However, if you have credits, your credits will offset the fees so you won't be charged anything.
You can earn more fee-free savings credits by referring friends.