Fiat currency is money which is issued by a government and which is not backed by a commodity such as gold or silver. Most global currencies are "fiat currencies," including the US Dollar, the Euro and the British Pound**.
Sling Money uses a digital currency called USDP as its main medium of exchange but allows you to display your balance and transactions in whatever fiat currency you like. This means your displayed balance may sometimes change as currency exchange rates fluctuate.
** For UK-based residents, Sling Money is only available in a Closed Beta.