This tool creates an Environment light and sets the Portal geometry parameter to the geometry object you select.
A regular environment light casts light on the scene from all directions. For an enclosed scene such as a room, where the "environment" light is outside and only enters through a few windows and doors, most of the light will simply hit the walls. The Portal geometry parameter specifies geometry (e.g. windows and doors) through which the light can enter. The environment light will only calculate light traveling through the portals, greatly improving efficiency.
Making an environment light that shines through portals
Select the object(s) you want to act as portals for the environment light, then click the Portal Light tool on the shelf.
With nothing selected, click the Portal Light tool on the shelf, then select the object(s) you want to act as portals for the environment light and press Enter.