You can think of a heat sink as a kind of a collector and storage for thermal energy. Ever noticed how pavement can get really hot during a hot summer afternoon, and yet it remains hot even as the sun goes down? The pavement (dark asphault being an excellent example) absorbs heat rather than reflecting it, and the denser the mass, the more it absorbs. As the day cools, the thermal mass gradually releases heat over time until it equalizes with the air.
Greenhouse operators often use barrels of water as thermal batteries (heat sinks) as a passive method to equalize day and night temperatures. In more complex geothermal setups, the ground itself acts as a heat sink and pipes are buried a few feet underground so air passing through can absorb the heat that's stored by the soil at that depth. Keep in mind, the denser the storage medium, the more heat it can store. Hot air holds much less energy than hot water, which in turn holds less than bricks.
