Short answers, pulled from the story.
Haitian Vodou is an African diasporic religion that developed in Haiti between the 16th and 19th centuries. It arose through syncretism between the traditional religions of West and Central Africa, particularly those of the Fon, Kongo, and Yoruba peoples, and Roman Catholicism brought by French colonists to the colony of Saint-Domingue.
The lwa are supernatural beings in Vodou theology, serving as Bondye's intermediaries with humanity. Vodou teaches that there are over a thousand lwa; they divide into groups called nanchon (nations), most notably the Rada and the Petwo. Each lwa has a distinct personality, associated colors, days of the week, and objects, and most derive their names from Fon and Yoruba languages.
Spirit possession is central to Vodou ritual. The possessed person, called the chwal or horse, is believed to have their gwo bonnanj displaced so the lwa can enter and communicate through them. The possession can last hours or sometimes days, and the chwal retains no memory of the events; through the chwal, the lwa offers counsel, warnings, and healing to the congregation.
Vodou teaches that the soul, called the nanm, has two parts: the ti bonnanj (little good angel), understood as the conscience, and the gwo bonnanj (big good angel), the psyche and seat of memory and intelligence. At death, the gwo bonnanj joins the ancestral spirits of the Ginen while the ti bonnanj faces judgement before Bondye.
Haiti's 1987 constitution enshrined freedom of religion, and President Jean-Bertrand Aristide granted Vodou official state recognition in 2003. Before this, the religion faced centuries of suppression, including the French Code Noir of 1685 that forbade open practice of African religions and Haiti's own government Operation Nettoyage in 1941 that destroyed many temples.
In Vodou belief, a zonbi is often described as the gwo bonnanj of the recently deceased that has been captured and forced to serve a master. The anthropologist Wade Davis argued this was rooted in a real practice in which Bizango secret societies used poisons to make individuals more pliant. Scholars have also interpreted the zonbi as a metaphor for the enslavement central to Haitian history.