Fiat stands for; let it happen, done or performed in Latin. Fiat is simply explained money without self-worth. It is money where the person who has "made" the money does not have the cover to redeem it at real / real values. Fiat money therefore only works on the trust / confidence that the next buyer in line will also accept that the money / banknotes will have the status of a means of payment.
Fiat money is what we today call money, and use for daily chores. They are accepted and used as a means of payment with the simple background and the belief that the public authorities for the currency in question will maintain the values of the "money".
Examples of fiat money are; Dollars, Euros, Norwegian kroner, etc.