German officials forced Jews into ghettos: designated, concentrated, and isolated urban neighborhoods, usually surrounded by walls or fences.
The very existence of European Jewish populations, demonized by propaganda, was 'a problem' for Nazi antisemites like Hitler, even before the ghettos. Classification, deportation, containment (segregation) in ghettos, and finally mass murder were 'solutions' to this 'problem' (which, tragically and ironically, was an intentional Nazi fabrication not actually a real problem).