In English, I think, it's akin to the imperative, but in Latin it's "hortatory." Or something. Maybe there's a little more passive voice in hortatory than imperative - "Let it be done!" rather than "Do it!".

"Fiat lux!" is the equivalent of "Let there be light!"

Do you suppose Fiat Motors, makers of automobiles in Italy, meant something by the name? As opposed to the Chevvy Nova in Latin America, which was teased for its alternative translation into "it doesn't go!"

Afterthought: Is "fiasco" simply Italian for "I'm doing it"?