Very simply, this is some receptacle that holds water. Generally speaking, they are immovable – a bucket could be a cistern, but that’s not how the word is used. A cistern is usually considered to a be a built-in feature of a building.
A cistern is filled, either by water delivery or by rainwater gathering. It might be raised from the building – or on its roof – in order to provide water pressure. A cistern is not open, because then the water would be easily contaminated and would evaporate.
Cistern water is not safe to consume directly. It would need to be filtered or boiled first.