>>34194588If I had to make a wild guess, since the game doesn't really explain it, is that it's the prism that contains Necrozma's consciousness/power and is able to transform & harness light. Its impaled into Solgaleo/Lunala when taken over, and when Necrozma is ready to Ultra Burst the light from the source is pulled through the prism, and the output results in the pure light for Ultra Necrozma.
