A light portal is a guide for photons coming from the environment. The bounding box around all light portals in the scene is used as a target for photon emission from an environment, allowing you to limit the photons to certain areas of the scene instead of shooting many more photons.
During still frame anti-aliasing all light portal materials are set to invisible in raytracing mode. In combination with Raytracing, this material only works in Full Global Illumination Mode. By default, objects with a portal Light Material do not affect the rendering in any other way.