Light Filters are used to manipulate 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.
Light filters must always be part of Maya's Default Light Set to work correctly. You check see this by expanding the defaultLightSet group in the Maya Outliner. Anything outside this set can be dragged into it to operate.
Creating Light Filters
To create a light filter in Maya, first select one or more RenderMan lights. Then right-click on the light filter button in the RenderMan shelf and choose a type of light filter. The light filter is created and wired into the selected light. As a convenience, the light filter is created at the same location as the selected light. Usually you will want to move the light filter somewhat forward from the light. The light filter is not parented under the light by default, but you may wish to do that if you want the light filter to always move around with its parent light.