Harrison Ford might not care in the slightest about what a Force Ghost is, but the apparitions of legendary Jedi are hugely important parts of the expanded Star Wars mythology. Not every member of the Order has the power to appear as a mystic spectral force, though, and in the movies at least, the honor has been reserved almost exclusively for the Jedi established as the most esteemed.
At the end of Return of the Jedi, Anakin Skywalker came back as a Force Ghost, which raised several questions after he spent the bulk of his adult life doing the bidding of the Dark Side as Darth Vader. In canon, the Sith can’t become Force Ghosts, and having spent a lot more time training under Darth Sidious than he ever did Obi-Wan Kenobi, it was never made clear how Anakin suddenly possessed the ability from almost the second he turned his back on evil and redeemed himself.
Even Qui-Gon Jinn couldn’t master it, and Force Ghosts can only be conjured by those who fully embraced the Light Side of the Force. Anakin is the only character to have shot to infamy as a Sith then later appeared in ghostly form, but a lot of it probably has to do with the fact that he was the Chosen One and thus the most powerful Jedi in existence.
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George Lucas explained it away by saying that he would have learned it from Obi-Wan and Yoda, which was then made canon by The Rise and Fall of Darth Vader novel, where Anakin’s master showed up in his dying moments to teach him one final trick.
As ScreenRant notes:
Darth Vader’s redemption and thus return to the light side as Anakin was the first step in making the Force ghost process a reality, but what came after that is still being debated among Star Wars fans. In the Star Wars: Return of the Jedi DVD commentary, George Lucas said Anakin was able to retain his original identity “because of Obi-Wan and Yoda, who learned how to do that”, suggesting he received assistance from them after he died, thus granting him the last step in becoming a Force ghost. This idea can be found in the now non-canon book The Rise and Fall of Darth Vader, where right after dying, Anakin is met by Obi-Wan, who tells him he’s in the “netherworld of the Force”, and if he wished to revisit corporeal space, he had one thing left to teach him.
Unfortunately, there’s still no official answer from Lucasfilm on this, meaning that like many dangling plot threads in Star Wars, it’ll be left entirely open to interpretation by fans.