The Spirit King (霊王, Reiō; ; ; Viz "Soul King") is the king of Soul Society, whose existence in Soul Society is as symbolic as it is absolute. The king resides in the Royal Palace that exists in a separate and special dimension inside Soul Society that is protected by the Royal Guard.The door to the dimension housing the Royal Palace is opened via the Ōken.
It's unclear what the Spirit King actually does, other than he has a specific system of "specialty duties" that are actively carried out. The duties apparently require a very high level of competency or skill of some sort. The Spirit King and the Royal Family are apparently completely indifferent to governing Soul Society; the Spirit King has given the full control of government of Soul Society to Central 46, thus being uninvolved with the day-to-day affairs of Soul Society.
According to a conversation between Kisuke Urahara and Sōsuke Aizen the Spirit King is detailed as both a "thing" and "it". Urahara details that without the Spirit King's existence, Soul Society would be rent asunder. The Spirit King is the "linchpin". If the linchpin is lost, then the world would simply crumble away and that is the way of the world.