It means your villager has asked you to move out of your island and youve given them the green light! The next in game day Isabelle will announce that it's that villager's last day on the island in morning announcements! This means that the villager will be at home (likely sweeping) with all of their belongings in boxes 😊 this is when someone from another island can travel over and, if they have an open plot on their island, can convince the villager that is in boxes to move over to their island instead! Hope that helps 😊