Light layers enable a rendered image to be split into additive "layers", in which each layer holds some contribution to the final rendered image.

How much a layer contributes to the final image can be adjusted interactively while rendering without restarting the process, and even after the render is completed, allowing for great flexibility in adjusting the lighting balance without having to do a lot of extra rendering. See Light Layers for more information on this subject.

The material's layer parameter specifies which light layer the (presumably light-emitting) material's contribution is added to.

Materials with attribute:
Glossy Transparent
Diffuse Transmitter