What is the meaning of the title?

The title of this novel is a common phrase that has been in widespread use for many years. The dictionary definition (from Google) of this seeming nonsense phrase is: "language or ritual causing or intended to cause confusion or bewilderment." This novel, experimental in its form and language, is indeed an exercise in, and commentary on, language and how it can cause confusion. The idea of "ritual," as mentioned in the definition, points to the origins of the phrase, which is also connected to magic, superstition, or ritualized objects used in religious ceremonies.
The central plot of Mumbo Jumbo concerns texts from ancient Egypt and attempts to share these texts, previously unseen, with people in 1920s Harlem. The overarching theme has to do with African and African American history, and the presence of the texts which are mysterious and indecipherable to many people is reflective of the lack of understanding of the complexity and vast scope of Africa's cultural history. This lack of understanding underpins racist attitudes toward people of African descent in the United States, and the novel seeks to comment on how such racism reflects ignorance.