When things sink, they settle or drop to a lower level. If your kayak has a leak, it might fill with water and sink to the bottom of the lake.
Things can sink, like a toy octopus descending to the bottom of the bathtub or the sun appearing to sink below the horizon. You can also deliberately sink something, as when you capsize your friend's remote controlled speedboat on purpose. When sink is a noun, it's the water basin in your kitchen or bathroom. The Old English root of both kinds of sink is sincan, "become submerged or go under," from a Germanic source.