Subsurface scattering (SSS) is an essential tool to make organic materials look real. They are probably most commonly used in human skin, but they pretty much apply for any living object, as well as some inanimate ones. Our skin consists of several layers, which all have different colors. Contrary to plastic they are also very heterogenous, meaning they consist of cells, blood, collagen and more. As a result light that penetrates the surface gets scattered.