I just read a joke and don't know what it means. "When the last person on earth was sitting alone in the room, someone knocked at the door." ...

This is science fiction, not a joke, but the shortest novel in the world at present. "The last person on earth" means that there is only one person on earth, but this is the door. Since there is no one but himself, what is that knocking at the door? Aliens or ... give readers a lot of imagination.