SSS, or subsurface scattering, is a visual effect that affects rendering of translucent objects. In such objects light is scattered beyond the surface of the object and exits the surface at a different angle (akin to refraction). This effect is very important to simulate some materials realistically.
With Minecraft the objects that usually take advantage of SSS are plants or leaves. Here’s an example of what can be achieved with the SSS effect.
Vanillaccurate now supports SSS !
This screenshot was captured using Kappa shader.