The US dollar was at one point backed by a gold and silver standard, meaning that you could always go redeem the cash for a specific amount of the metal. After the Great Depression the US moved away from the standard, backing the money with the faith in the US government.

This is why each note has “Backed by the full faith and credit of the United States” printed on it.

https://en.m.wikipedia.org/wiki/History_of_the_United_States...

https://www.reference.com/business-finance/backs-up-currency...