A cistern is an underground tank that holds water. A long time ago, cistern water was used for drinking, but nowadays developed parts of the world mainly use it for watering plants and flushing toilets.
The oldest cisterns that we know of were built in the Middle East about 12,000 years ago, which is around when farming was invented. That's no coincidence. It would be tough to collect enough water to grow barley and wheat without storing the water in something, especially in a place as dry as the Middle East.