What does the word ghetto mean in holocaust time?

Same as it did before and the same as it does now. It is an area of a town or city where a specific ethnic group dominates. It is not a reference to socio/economic status, only ethnicity. For instance, those who live in Southside Chicago are largely Hispanic and the area is a ghetto. Central south Grand Rapids, Michigan is a center for Asian, further south is an Arabic area, a little north are Hispanic and African-American neighborhoods, while the Northwest has been traditionally an enclave for Polish immigrants, while Ada and Lowell are white, European-American suburbs; these are all ghettos. Some are richer areas, some are poorer, most are really middle class. All that matters is that the majority of residents in these areas are generally of one ethnicity.

The term has been used derogatorily since at least WWII or a short time before, largely because of the treatment of Jews in the ghettos in Warsaw and other Polish cities.

It entered the language in the early 17th century in Venice, Italy. It referred to an island where Jews were forced to live. It was derived from the verb ghettare (to throw), and literally means foundry for artillery (the name of the island).