View Sub-surface Scattering
Sub-Surface Scattering (SSS) is a shading method (shader) used by the rendering engine to reproduce a more natural light scattering effect that most things in nature tend to have. The easiest example of this would be to hold your hand up to a bright light source. Notice the how your skin illuminates? In some cases, you may even be able to make out some internal blood vessels and perhaps even the shadow of bone—this is sub-surface scattering in a nutshell.
SSS can be seen in every living thing, from animal to vegetable, fruit, etc. You can see it in grapes and cirtus fruit, the ears on your pet, the petals of flowers and so on. Use of SSS will completely change the feel of your rendered imagery, and bring a sense of beauty and realism to your otherwise “dead-looking” renderings.
Not limited to natural materials, SSS is also common in many manufactured materials as well—most notably waxy substances, such as plastics. SSS tends not to occur in metals.
In actual execution, the renderer considers light scattering through the actual material of the object, and represents that in the final image. Light is followed from the contributing lights in the scene (you can control which lights add to the effect), all the way to the surface and sub-surface of the object. The result is then recorded and imaged by the render engine.
For the artist, there are many controls available to fine tune the look desired. You can control the color of the scatter; the depth to which light will penetrate (Path Length), the strength of the effect, and even drive or modify the effect with procedural shaders and image maps.
SSS can add much to render times, especially when sampling levels are at higher quaity settings. That said, it would be difficult, though not impossible, to acheive mose of what you get from SSS by more traditional CG lighting methods (which are typically in the realm of very experienced old-timers).