Hearing that title makes longtime X-Men comic book fans instantly geek out; however, fans who are only familiar with the X-Men movies may not understand what all the excitement is about. So, if you don't know the long and iconic history of The Phoenix entity in Marvel Comics, we're here to help.
Here is X-Men's Dark Phoenix Explained.
The Phoenix Force
The first thing to know about The Phoenix is that is not just a person - it's a cosmic entity of unspeakable power. It has a lot of grandiose descriptions ("primal force of life and passion"), but it is the center of all psionic energy in the multiverse (the energy that psychics tap into). When we first meet the Phoenix in X-Men lore, it is the protector of the M'Kraan Crystal, the nexus of all realities in the multiverse.
The Phoenix used to roam the universe, pruning the galaxy according to its own judgement (i.e., destroying and re-growing entire portions of the cosmos). However, it has a history of being influenced by Earthlings - gaining its very Phoenix fire bird form from an ancient magician.
But when it met a certain mutant, everything changed, forever...
The Dark Phoenix
The Phoenix Force hit a milestone when it came into contact with the X-Men's Jean Grey. After helping the X-Men save the M'Kraan Crystal, The Phoenix stayed bonded to Jean - but eventually that bond became something dangerous. Thanks to The Hellfire Club's manipulation, Jean/Phoenix's mind was corrupted, and she became The Dark Phoenix. It was believed that Jean ultimately sacrificed herself and took the Phoenix with her - but both were eventually resurrected.
Now, the Phoenix Force exists in the Marvel Comics universe, showing up at key junctions in various storylines, having taken on new hosts that include the Mutant Messiah Hope Summers, Emma Frost, and even Cyclops. In the "Avengers vs X-Men" storyline, Cyclops and Emma Frost actually go to war with the Phoenix Force, with Cyclops killing Prof. X, while possessed by Dark Phoenix.
Ultimately, Hope Summers and Scarlet Witch wished away the Phoenix Force and restored the X-gene in the process, allowing for the creation of new mutants, after they nearly went extinct.
In The Movies
In the X-Men movies, the Phoenix Force has already appeared as a manifestation of Jean Grey's powers. It first was hinted at in X2, when Jean seemingly died; in X-Men: The Last Stand, a resurrected Jean became Dark Phoenix. The Last Stand borrows heavily from the storyline where Jean ultimately sacrifices herself, with Logan delivering the mercy killing (an event which inspired The Wolverine and Logan).
However, in the new timeline established by X-Men: Days of Future Past, Jean has started to manifest the Phoenix Force at a much younger age. She unleashed her fiery power against Apocalypse in X-Men: Apocalypse, destroying the ancient villain; but it should be noted that Apocalypse didn't fear The Phoenix - in fact, he felt his prophecy of a mutant evolving to a new level of god-like power was vindicated by his encounter with Jean.
In X-Men: Dark Phoenix, it remains to be seen what happens with Jean's considerable powers - and if The Phoenix Force will have the same connections to cosmic forces that it does in the comics.