In Project 11, subsurface scattering is added to the renderer. This implies that light hitting the surface of the material enters and bounces around inside the medium before exiting. Many materials such as grapes, skin and marble exhibit this quality. Subsurface scattering in this implementation used ray marching, where the ray enters the medium and bounces around until exiting. Because the ray actually bounced around inside the object instead of just off the surface, the rendering was far more computationally expensive than rendering this scene without SSC.
In the image below, two spheres are placed side by side with a large rectangular light source placed overhead. The sphere on the left uses subsurface scattering while the sphere on the right uses just Lambertian reflections. Light shining from above simply bounces of the top of the Lambertian sphere, leaving the bottom dark and unilluminated. The SSC sphere, however, has light shining through the medium and appearing at the bottom of the sphere as many materials would