The water purifier is used to purify salt water into drinkable water. It works similarly to the cooking station. To purify water, put salt water in the hotbar, select it, then press the use key on the water purifier. A fire will start and the water will boil for a period of time until it is done. When it's done, the fire will go out the player may collect the drinkable water.
The water purifier seems to be made of a metal pot with fire logs in it and hole in the side for air flow. A grill rests over the pot along with a container. Attached to this container is what appears to be a piece of metal, which would catch and condense the steam from the boiling water. The clean water would then drip into the empty tin can, supported by some fastened sticks.