Someone who is improvident doesn't worry about the future — or plan wisely for it. If you spend all your money on video games even though you know you have to buy your mom a birthday present next week, you have made an improvident decision.
In the adjective improvident, the prefix im- means "opposite" or "not." Provident comes from the Latin word providere, meaning "foresee, provide." Put that together and you get something that lacks foresight and consideration for the future, like an improvident town council that spends the whole budget on playground equipment, leaving nothing to cover the inevitable repairs to fire trucks or public restrooms.