Rukongai in the Manga and anime is the area outside of Soul Society where most of the souls reside after a soul burial (a process similar to sending a soul to heaven) or purification (the cleansing of the soul after it grew into a hollow). It is comprised of eighty districts; the high the number of the district, the more violent it is.
Many of the souls in Rukongai aspire to become shinigami, and some of them, such as Abarai Renji or Zaraki Kenpachi make it.
Rukongai in Legend of Bleach
Rukongai seems to be a central location for many of the players. It is also a starting location for several of the shinigami races.