Light Filters are used to manipualte lighting results. In the real world this is often achieved through the use of stencils and gels to change the color, quality, and amount of light in a shot. RenderMan's light filters are a powerful and flexible way to do the same (and even more) in lighting your scenes.
PxrBarnLightFilter allows us to create physically accurate window barns to simulate the real set lighting with correct shadowing. Its other usage include controlling bounce lights in a large scene. In addition to the physical mode, it also provides an analytic mode.
PxrGoboLightFilter projects a painted texture in front of the light.