The sink is a piece of furniture that can be built in the Kitchen of a player-owned house with level 47 in the Construction skill. When made, it grants 300 experience. Players can use an empty bucket or other such item to receive water from the sink. Building a sink requires at least 47 Construction. There is no practical use of having this piece of furniture in one's house over a pump and drain, other than looking fancier.
Similar sinks are also found commonly around RuneScape, and can be similarly used as a source of water. Their examine text is "Running water - a nice feature". The sink in the Crafting Guild reads "Wash your hands!"