The Heaven Realm is a plane of existence where the souls of those who had done good in their time while they were alive go to after they die.
The Heaven Realm although never seen completely appears to be a large infinite plane of existence completely covered with nature resembling a massive garden with a super giant star sized palace behind a large gate guarded by a single Angel named Petr. When one dies they are sent to the gates where it is decided if they proceed through the gates to the Heaven Realm or if they descend into the Hell Realm for the rest of eternity. The ruler of the Heaven Realm is an all powerful being addressed as "he" or "him".
Residents in the Heaven Realm appear in their soul form as they had lost their physical body at their death. However they can still transform into the image of their living self when interacting with other souls, except in a blue tint of color.
Returning to the Living Realm
For the most part souls are not allowed to leave the Heaven Realm without special permission from "Him". However those with powerful souls such as Xaya are able to come and go as they please, although they are still punished in some way shape or form depending on how badly they interfered with the Living Realm. There are also Realm Portals that Angels use to travel to the Living Realm.