Itachi Uchiha's soul returning to the Pure World.
The Pure Land (浄土, Jōdo) is the afterlife in which the souls of earthly beings generally reside in death. It appears to be a plane of nothingness, where not even consciousness exists. A soul that was revived from the pure land doesn't remember being there, its last memory being the one it had right before its death. Souls that have been sealed, such as by the Dead Demon Consuming Seal, cannot enter the pure land until the seal has been released. Furthermore, only those souls that reside in the pure land can be reincarnated with the Summoning: Impure World Reincarnation.
To arrive at this place, a dead person has to go through another plane of existence, where other people such as relatives or friends can be met. People residing in this plane are those who haven't been able to move on because of earthly regrets or because they decided to wait for someone. Obito noted that chakra is the power that connects this plane and the real world, and as such he was able to teleport back to the world of the living to meet Kakashi once more.
Trivia
- Hagoromo Ōtsutsuki had called upon the spirits of the previously deceased Kage from the Pure Land to help with the summoning of Team 7 and the tailed beasts from Kaguya's Dimension.