What are FIAT currencies, please? A short story

Fiat money is a currency that has no internal or fixed value and is set by law as a means of payment. Traditionally, currencies based on physical commodities such as silver and gold, while the Fiat money based on the credit of the economy. The value of money depends on supply and demand and was introduced as an alternative to commodity and representative money. Commodity money is made from precious metals such as gold and silver, while representative money is a claim on a commodity that can be redeemed for cash.