When water gets very hot, it boils and turns into steam. This happens at 212 degrees Fahrenheit or 100 degrees Celsius. The water turns into steam because you (or something else: in the picture it is the volcano nearby) is adding energy in the form of heat to the water.
As the water molecules get more energy from the heat, they move faster and faster. When they get enough energy, they jump right out of the water and into the air. Once they are in the air, still hot and moving fast, they bump against each other and other molecules in the air so much that they quickly move apart from each other. That’s why steam takes up more room than water.