Can someone please explain the rewinding part🙏

Ahh nothing much. They just died in their 2 past life. 1 where Sa young died and 1 that J died. They rewind it because they wanted to save the earth as well as each other. So this timeline is their last and 3rd rewind. But for some unknown reasons, past Sa young didn't merge to the current one so basically, in Sa Young body, it has 2 Sa Young. Just read it . it's pretty good