The RR is The Reincarnation Realm. Where you go to get reincarnated. Although the name may seem like it's a good thing, don't be fooled. It's more of a punishment for not being able to get life right the first time.
Once you get reincarnated you go through the Saṃsāra cycle (Sanskrit, Pali; also samsara). I guess the Buddhists were correct about certain things in the after life. Samsara is the beginningless cycle of repeated birth, mundane existence and dying again. This cycle is considered to be filled with suffering and pain.
It's a well known fact amongst the gods of death that The Reincarnation Realm is the worst realm of all.
