"Antebellum," spelled variously as ante-bellum, ante bellum and antebellum, is an English borrowed word from the Latin ante bellum, which is two Latin words that together mean "before war": ante, before; bellum, war. In American English, antebellum has come to mean before the American Civil War.
Therefore, in American English and American History, antebellum refers to the time before the Civil War broke out. This includes allusion to a pampered culture and to plantations that flourished and did so because of slave labor and slave-backed culture, as nothing in Southern culture could have existed or operated as it did were it not for the domestic and agricultural and others kinds of servitude of enslaved Africans.
So in Jubilee, antebellum in the Part I title, "Sis Hetta’s Child—The Ante-Bellum Years," tells you that the story opens in a time prior to the beginning of the secession related American Civil War. The title also tells the main characters (Hetta and her child) along with the time period of the setting.