Evaporation happens when the sun's heat causes the water molecules to move and vibrate faster. The faster they move, the more likely they will escape from the liquid state and become a gas. The water vapor then rises into the atmosphere, cooling and condensing to form clouds. The clouds release the water back down to the Earth's surface as rain, which can return to the oceans and lakes. The cycle repeats.
Evaporation is essential for many reasons. It helps to regulate the Earth's temperature. It also helps to distribute water around the planet. And it is essential for the formation of clouds and rain.