There are some aspects of the original Star Wars trilogy that took decades to receive an explanation. Based on the events of the first set of films, it seemed clear that masters of the Force gained a sort of immortality in the form of a Force Ghost, with every Jedi slain in the trilogy returning from the afterlife. But in subsequent entries in the franchise, the death of Jedi did not result in the same phenomenon, leaving many fans scratching their heads.
The first sign that there was an existence after death for Jedi came in A New Hope when Obi-Wan Kenobi warned Darth Vader about returning from the grave if his former padawan was to strike him down. His words proved true (from a certain point of view) as his body disincorporated upon Vader slashing him while his voice returned to guide Luke Skywalker in the destruction of the Death Star at the film's end.
Obi-Wan's Force Ghost went on to give Luke further guidance, leading him to Dagobah to train under Yoda in The Empire Strikes Back. Yoda himself and the rejuvenated Anakin Skywalker appeared as Force Ghosts right alongside Obi-Wan at the close of the trilogy, appearing at peace and content with the bright future ahead of the Galaxy. From that point on, however, Force Ghosts rarely appeared in canon stories.
The prequel films saw countless Jedi slain. Not only did none of their bodies disincorporate upon their deaths, but none of them returned to offer guidance or wisdom from beyond the grave. It seemed clear that the explanation for Force Ghosts was not simply that it was a natural consequence of harmony with the Force, but that it was a skill only available to very powerful users. Even then, it did not seem to be a question of power or harmony alone as Mace Windu and several other Jedi Masters killed during the prequels never returned as Force Ghosts.
To find the explanation for how the mechanic of Force Ghosts works, however, one would need to turn to one of the prequel's most famous masters who actually did return from the afterlife. Although none of the main films featured it, the ability turned out to be a unique talent of Qui-Gon Jinn's, slain at the hands of Darth Maul in The Phantom Menace. Qui-Gon would go on to appear in The Clone Wars, offering a deeper insight into how immortality for the Jedi operates.
The question proves the central focus of the three-episode arc that closed out the sixth season of The Clone Wars. It all started in the episode "Voices" when Yoda began hearing Qui-Gon's voice calling out to him. When Yoda expressed his concerns to the other Jedi they expressed concern for the elderly Jedi -- after all, they explained, communication from beyond the grave was impossible. Resolute in the face of their concerns, Yoda set off to the Dagobah at Qui-Gon's behest to find answers.
Yoda's journey eventually brought him to the heart of the galaxy where he discovered the secrets that Qui-Gon kept to himself all those years ago. There, at the birthplace of midi-chlorians, it was possible for a Jedi to endure three trials that would allow them to retain their identity even after death. Although Qui-Gon did not fully complete the trials, and thus did not fully master them, he still retained part of the ability.
Ultimately Yoda resolved to do the same. While the trials proved grueling, forcing Yoda to confront truths and weaknesses about himself he needed to accept and let go in order to enhance his connection with the Force, he returned to the Jedi Temple to report that he did not find anything of consequence. While Yoda seemingly kept the secret to himself for some time, it's clear from what followed that he passed his secrets on to others.
Since The Last Jedi showed Luke communing with Yoda's Force Ghost, it even makes sense that the Jedi Master passed his secrets on to his pupil. Luke's body disincorporated into empty robes upon his death at the end of the film, and The Rise of Skywalker showed that in the Force no ending is ultimately final. Yoda and Qui-Gon may have held onto their secrets during their lifetimes, but in passing them on, they ensured that striking down any Jedi could mean making them more powerful than anyone could possibly imagine.
Streaming on Disney+, the final season of Star Wars: The Clone Wars stars Matt Lanter as Anakin Skywalker, Ashley Eckstein as Ahsoka Tano, Dee Bradley Baker as Captain Rex and the clone troopers, James Arnold Taylor as Obi-Wan Kenobi, Katee Sackhoff as Bo-Katan, and Sam Witwer as Maul.