what do you mean by emissive material ? What are those ?

Adrian is exactly right. With Blender materials we can use the emissive property to tell an object's surface to emit X amount of light.

That may seem very odd to make grass or leaves emit light in the context of this project. In this case we're using the emissive quality at a low strength to manipulate the shading of the object (namely eliminated accurate shadows). This gives us fine-tuned artistic control at the cost of physical accuracy. It's all for the sake of achieving a visual style that's difficult / impossible to achieve with accurate lighting.