"Sinks are plumbed and do not need to be manually emptied or refilled."
The Sink is a Sanitation/Medicine building in Oxygen Not Included. Duplicants use the Sink to wash Germs off themselves in much the same way they would a Wash Basin, except that the Sink requires Plumbing. The intake brings in clean Water, and the output removes Polluted Water.
The Sink uses Water at around 5kg per use and outputs Polluted Water. Duplicants will use the Sink when passing by it in the direction the arrow above it points. Place these near the Lavatory or anywhere else Germs may accumulate and transfer to Duplicants.