Federation credits are units of currency used in transactions with some Federation and non-Federation worlds or for goods and services that cannot be replicated efficiently. Although not a monetary unit in the classical sense, it makes exchanges with alien planets possible. (TOS: "The Trouble with Tribbles"; TAS: "Mudd's Passion"; TNG: "The Price")
The Federation would have payed 1.500.000 Federation credits as a lump sum and then 100.000 credits every Barzanian year for the rights to the Barzan wormhole. (TNG: "The Price")