Steam coming from underneath the hood of your vehicle may mean that

Explanation

Steam coming from underneath the hood may mean that the cooling system has overheated. Pull over to the side of the road and turn off your engine immediately. It's a good precaution to carry extra coolant or water in your car to add to the radiator after it cools down. Modern cars typically require a 50/50 mixture of coolant and water, so pouring only water into the radiator may lead to the engine overheating again eventually. If all you have is water, add it to the radiator, but also add coolant as soon as possible.