The Water Pump is a structure, used to extract fresh water from a water well. They are common throughout Chernarus and can be found in the middle/larger cities, and in the countryside on the outskirts of small towns and villages.
As with any other fresh water source, it can be used by survivors to replenish their thirst, or use to refill Canteens and Plastic Bottles. In order to refill your Water Bottle or Canteen at these locations, you must first equipping the container in your hand and when looking at the water pump and option will appear to fill the container.
Also, the water you get from pumps is always safe, meaning you cannot get an infection from drinking it. However, drinking too much water too fast will make you vomit.