There is a name for this type of stamp that I can't recall right now.
Basically, in foreign countries, there are people who will authenticate that a bill is real, and not counterfeit. They get paid a tiny percentage of the bill's face value for authenticating it. They stamp the bill with their own mark when they are done. I have been told that if they authenticate the bill and it turns out to be counterfeit, they pay for the bill's total face value, but I don't know if this is true.
Let's say you live in a remote African village, where US currency is still considered the king of currencies, and someone wants to pay you in US bills for your work or goods. The only problem is that you've never seen but a handful of US currency in your life...but you know that if you get paid in US dollars, you can exchange it for a premium for the currency you want.
You'll need someone to authenticate it, and stand behind the fact that the bill you have is real.
Look at a stack of $100 bills sometime. Generally, they have these little stamps all over the backs of them.