The answer in simple terms is the process of changing sea water into drinkable fresh water. This means taking out the salt and any bacteria and pollution. This process is now being used all around the world to provide people with a much needed dependable supply of fresh water.
In ancient times, many civilizations used this process on their ships to convert sea water into drinking water.
In nature the sun causes water to evaporate from surface sources such as lakes, oceans, and streams. The water vapour eventually comes in contact with cooler air, where it re-condenses to form dew or rain. This process can be copied artificially more rapidly than nature, using man made processes of heating and cooling.