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, albeit not physically.
About this creation
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, perhaps with the destruction of the Jedi library world of Ossus in the Exar Kun Wars or the First Jedi Purge. However, it was secretly preserved over the millennia by the native Ysanna, descendants of Jedi trapped on the world and slowly driven to barbarism.