For example let's say Tia moves from your town without anyone adopting her. (if your villager is packed up and about to move someone can come to your town and ask them to move to their town) Since she had nowhere to go but still moved out then she goes to your "void" which basically means that if you wifi with someone that has less than the maximum number of villagers (max is 10) then she would move into their town. The only way to make sure Tia moves into no one's town is by visiting someone with 10 villagers. That "clears" it out for you. Most people don't want a random villager moving in so they try to clear their void.