You take a parabolic concentrator and focus some sunlight, optically split with plastic fiber into visible light and heat. Pipe the visible light through diffusers throughout the building. It saves lighting electricity, of course, but unlike skylights or conventional T8s, it adds almost no heat to the building. In a cooling climate it saves about a third as much in air-conditioning energy as it does in light.
You still need electric lighting supplementation, but in office buildings you get most of the light when you need it. And the advanced diffusers means you have only one hole to cut into the building. That gets you better odds of decent installation that avoids leaks or thermal bridges.
Unlike solar PV, which typically turns 15 percent of sunlight into electricity, this system saves the energy equivalent of about 50 percent of the energy striking the collector. Solar heating can do better, of course, but in cooling climates you don’t have much call for that, and hot water demand is a much smaller percentage of total demand in office buildings than in residential ones. Originally the intent was to convert the other 50 percent of energy not used for lighting into electricity. Apparently the added capital costs were too high for deployment in actual commercial use.