Bleach operates on the idea of reincarnation. If you die in the Human World you go to the soul society. When you die in the Soul Society you go to the Human World. Note: It has been also mentioned in the manga that a shinigami's body is nothing but their soul. When their soul "dies", the "body" becomes part of the spirit particles that are soul society. This is mentioned in manga 268 by Kaien in Rukia's flashback. It is not made clear however, what the general process of reeincarnation is in soul society, only that there has to be a balance between soul society and the human world.