Most divination is private. Somebody has a problem, goes to a babalawo, pays, and receives a reading and a prescription for their own life. The Araba is the office that exists because some questions are not private. As the senior babalawo of a town he casts on the town's behalf: at the ritual year's public castings, at the installation of a king, and when something has happened that concerns everybody.
The office is a chieftaincy inside the priesthood rather than an appointment from outside it. It is filled from among initiated babalawo by the priests themselves, and it carries duties beyond casting: the training and examination of younger diviners, adjudication when two diviners disagree, and the ordering of the priesthood's own affairs. Several towns have an Araba and the towns do not answer to one another.
At Ile-Ife the holder is styled Araba Agbaye, which is usually rendered as Araba of the world. The claim in the title is a real claim and it is not universally accepted, which is a separate matter from the seniority of the Ife office within Yorubaland itself. Within Ife it is not in doubt.
One thing the office is not is a priesthood of an orisha. An Araba is a babalawo, initiated to Ifa and working the corpus, and the cults of Sango, Osun, Obatala and the rest are headed by their own priests, who answer to their own lineages and not to him.