Gold Standard, Government Protection, Debt, Government credit instruments.
But everything I read about currency tells me that its largely always just agreed upon make believe. The case with the Real absolutely confirmed that for me.
The benefits of having a common/standard unit for an economic ledger are enormous, as it greatly reduces trading friction. That added benefit has real (i.e., not make believe) value, and the more liquid the currency the higher the monetary premium.
Even after hyperinflation, if you normalize to the money supply, currencies generally don't lose value without losing liquidity (being less "agreed upon") .. "Inflation is always and everywhere a monetary phenomenon"
And, conversely, if you try to pay with cash to some shady people in a parking lot, there's not much good you're going to get out of courts if they defraud you.
Just like currency. All of those were imaginary since they were invented. People have create physical representations for currency in particular, but that doesn't change the fact that it's only a group of people agreeing on something that only exist on their heads.
Equally imaginary things are "corporations", "governments", "military"... If you push hard enough, even "person" fits that, but this one is deeply entrenched in people's minds.