The lights are similar to the threads of light that connected Severus and Narcissa when they performed the Unbreakable Vow.

Harry's Prophecy is tied to all the people - he is expected to behave as if he had performed a Vow because the Prophecy supposedly binds him.

When he decides he won't, that he'll defy the Prophecy, those connecting threads unravel - hence the lights.

Thanks for reading! ♥