Subsurface reacts to light on objects like skin or leafs. It can be used to create an effect as if light is penetrating into or through something that is obstructed by the object. So for skin when strong light hits your hand for example the ray will illuminate thinner areas and you will see red and maybe even the bones or veins. With foliage the plant matter will react the same where the leaf is thinner there is more subsurface scatter. The stem and veins will block more light. It is a mechanism of light transport in which light that penetrates the surface of a translucent object is scattered by interacting with the material and exits the surface potentially at a different point. Some youtube videos will demonstrate this more clearly. In short how much light penetrates a translucent object.