A ghetto is an area of a city where minority members of the population gather together and live. Often these citizens are low-income, perhaps immigrants, and marginalized by the majority culture. In contemporary times, you may be familiar with the term "ghetto" used to describe people or items in a negative way, such as, "This school is so ghetto. Our books are ten years out of date."
In Elie Wiesel's time, Jews were segregated in separate areas of cities. This forcible segregation of Jews was not new during World War II. It had occurred for centuries in many countries. In fact, the term "ghetto" is Italian.