Uta is a movie villain so there actually isn't as much lore for her compared to main series antagonists.
Johnbobb posted...
Her actual plan beyond "punishing them for the their love" isn't entirely clear within the song itself, but it certainly sounds violent.
Would you believe me if I told you her plan is similar to Maruki's? Basically she hosts the largest livestream ever, uses her devil fruit to bring everyone that tunes in to her Sing Sing World, and then plans to let herself die IRL so that everyone is trapped there forever, creating a dream world that is an endless concert. She actually describes her plan in the opening song, New Genesis, but that's a much less villainous song and it's more meant to be foreshadowing than her actually saying the plan outright.
Anyway Backlight plays when the Straw Hats don't want to go along with that and first come into conflict with Uta.
It's tempting to linger in this moment, while every possibility still exists. But unless collapsed by an observer, they will never be more than possibilities.