Subsurface scattering
Light that enters a translucent material, bounces around inside and exits somewhere else — what makes skin, wax and marble look alive.
An ordinary surface model lets light bounce right where it lands. But in skin, milk, marble and leaves the light penetrates, scatters beneath the surface and leaves it somewhere else. Hence the red glow you see in an ear against the sun or fingers around a flashlight. Without the effect, computer skin looks like plastic.
Jensen et al. formulated the physical model in 2001 with a BSSRDF and a dipole approximation. Real time makes it cheaper: the diffusion is approximated in texture space or screen space, where Jimenez's separable filter (2015) became common in games. The effect belongs to the same family of indirect-light phenomena that radiosity and photon mapping try to capture at the scene level.