The Spirit World is a parallel plane of reality that coexists alongside themortal world, and is home to the spirits, immortal supernatural entities which often embody different aspects of life and nature.
Prior to 171 AG and the opening of the spirit portals, it was extremely difficult and rare for any human, other than the Avatar, to enter the Spirit World; conversely, only a handful of spirits were capable of existing physically in the mortal world, and doing so required them to give up their immortality. The Spirit World and the physical world are closest together during the solstices, which made it possible for spirits to manifest in the mortal world and for humans to pass into the Spirit World. There are also certain locations in the physical world which act as bridges to the Spirit World; these locations are often revered by the cultures living near them, such as the Spirit Oasis in the North Pole. The Avatar acted as a spiritual bridge between both worlds and can travel between them freely.Individuals who cross over to the Spirit World cannot use their bending, unless they have entered physically through the spirit portals. Despite this, the Avatar is able to use energybending when in the Spirit World, even when they did not enter physically, as they are connected to all the spiritual energy when in that realm. The portals were reopened in 171 AG, and following that year's Harmonic Convergence, Avatar Korra decided to leave them open for good.
There are few known ways for a mortal to enter into the Spirit World, though humans are sometimes carried there by a spirit as the entity shifts between planes. Enlightened individuals can cross over to the Spirit World by meditation and others can enter physically through one of the three spirit portals.