The Un-People is a segment in Between the Lions. The sketch is about prefixes "un-" and "re-".
Monica Maxwell is a girl who does a certain task (example: buttoning her jacket). But then the evil Un-People come to undo it (unbuttoned). But then the super heroic Re-People appear, and "re-do" the task (rebuttoned).
The sketch is considered a parody of superhero cartoons, like the Justice League.