The Force ghost was the soul and essence of a deceased Force-sensitive who denied the will of the Force upon death, yet was able to interact with the living. It also described those who had come to terms with death and invoked this technique to preserve their identity, manipulating their own pneuma to guard against being absorbed into the Force in such a way that they would lose their identity. Some Sith Lords learned similar techniques, which in some cases were combined with the art of Essence Transfer, and thus allowed them to physically interact with their environment. This ability also allowed the Force user to interact with both living beings and other Force ghosts.
According to the Jedi tradition, death was a part of life, and it meant becoming one with the Force. In death, sentients lost their ability to communicate with the living, but mastery of certain obscure paths of learning could avert it.
The origins of that knowledge, as well as its history, are not known. Ancient Jedi of the Galactic Republic such as Arca Jeth, as well as Sith Lords, knew a secret which could enable the user’s essence to «survive death», but this knowledge was somehow lost over the millennia. However, it was secretly preserved over the millennia by the native Ysanna, descendants of Jedi trapped on the world and slowly driven to barbarism.