The main characters created a metaphysical realm to find each other after their deaths—a place producers and fans called the "flash sideways" world. Though they all died at different times, the characters appeared in the realm together, letting them meet and remember the "most important part of their lives": their time together on the Island, and then finally "move on".
CHRISTIAN: I'm real. You're real. Everything that has ever happened to you is real...
Jack on Oceanic Flight 815 in the flash sideways world.
Much like the Island itself, this realm let them reflect on their lives and revisit past choices. Experiences during the flash sideways—pain, unconsciousness, brushes with death and deja vu—helped them recall their lives. Connecting with something particularly significant, often a loved one, brought back all memories and let characters "move on" to the next world.
Helen speaks with Locke in the flash sideways world in ("The Substitute")
Besides the main characters, characters such as Eloise arrived in the flash sideways after their deaths. Thousands of others populated the world as well. The show leaves unclear though whether they too died in the real world or whether the main characters created them like they created the world itself. Some of these people—David, Sam, Eva—definitely did not exist in the real world. ("Lighthouse") ("Sundown") Some, such as Helen and Nadia, affected the main character's real lives greatly. ("The Substitute") ("Sundown") A few, like Kim Kondracki and Lynn Karnoff, played minor, arguably forgettable roles in the characters' lives. ("What They Died For") ("The Substitute")
Not all main characters entered the flash sideways. Eko died completely comfortable with the decisions he had made. Michael remained on the Island, whispering, and Walt later came to the Island to help him. ("Everybody Loves Hugo") ("The New Man in Charge")
Though many characters' circumstances matched their pre-crash lives, most retained the development they had achieved during the series.
- Ben acts dramatically different from how he did before the crash, possibly following years of redemptive life on the island post-series. ("Dr. Linus") ("The End")
- Hurley was a philanthropist, reflecting his years of caring for the island. ("Everybody Loves Hugo") ("The End")
- Jack believed nothing was irreversible, as the last days of his life had made him a man of faith. ("LA X, Part 2")
- Kate sported an affinity for motherhood that she had never had pre-crash, suggesting to Claire that she keep her baby. ("What Kate Does") ("Eggtown")
- Locke expressed the confidence and spirituality he had gained on the Island. He remained obsessed with his father though, and he regressed when with Randy. ("LA X, Part 2") ("The Substitute") ("The Candidate")
- Sawyer followed a career in law enforcement as he had in DHARMA, a complete reverse of his pre-crash life. ("Recon")
- Sayid, always an atoning killer, seemed to become even more of one following the actions at the end of his life. However, at least two of those killings were in legitimate defense of his life. ("Sundown")
- Sun and Jin had a better relationship than that which they started out with in the original timeline. ("The Package")
Christian reunites with Jack in the flash sideways world in ("The End")
According to Christian, the characters created the flash-sideways world to "find one another". Finding one another let them "remember and to let go". To this end, the characters ran into one another repeatedly after a series of coincidental circumstances. Eventually, characters connected with someone or something that let them fully recall their lives.
Before meeting one another, the characters imagined new situations for themselves. Director Jack Bender describes this other life as letting characters see "what you wish for or what you're scared of." (Lost: The Complete Sixth Season (DVD))
Passage of time
According to Christian, "There is no 'now'" in the sideways flashes. Character interactions do not exist on a single time line. For example, between Flight 815's landing and Keamy's death, Sun and Jin experience a single night, but Sayid experiences at least two. Other characters go through an entire week during this time. Characters experience more than a week between when waiters lay tables for the climactic concert and when the actual concert occurs.
Mikhail is killed...again? ("The Package")
Characters such as Mikhail Bakunin and Martin Keamy were showed to have be killed in the flash sideways world, suggesting those in the flash-sideways were not immortal. Given the nature of the world as a form of afterlife, it is unknown what would become of individuals who experience a second death, or if they were the real versions to begin with, as the show chooses not to dwell on it.
Red herrings initially suggested the flash sideways world was not a form of afterlife.
Edward Mars escorts captured fugitive Kate Austen. ("LA X, Part 1")
See also: Timeline differences
The flashes' first appearance immediately followed a flashback to Jughead's apparent detonation. This suggested that though the 1977 characters traveled to 2007, the detonation also somehow created a separate timeline. ("LA X, Part 1") Later episodes alternated between the flashes and the main timeline, suggesting two timelines ran concurrently.
The semi-canonical Lost Untangled recaps claimed that the detonation created the flash sideways as an alternate timeline, one in which the survivors successfully reset time and Oceanic Flight 815 landed safely. Sawyer had believed the reset would send them to LAX. ("The Incident, Part 2") Faraday in the sideways flashes later theorized that a nuclear blast had created his world. ("Happily Ever After")
Despite these red herrings, the producers refused to call the flashes an "alternate timeline". (Official Lost Podcast 2/22/2010) The finale finally explained the flash sideways, completely debunking the alternate universe theory. The question of whether or not the blast contributed to the sideways world's creation remained perhaps deliberately ambiguous.
Man in Black's construct
In the beginning of the season, the sideways flashes revealed the island underwater. The Man in Black later attempted to "destroy the island" by sinking it, but the link turned out to be a red herring. In "Sundown" the Man in Black offered to give Sayid "anything" he wanted, including the woman who'd died in his arms. The episode juxtaposed this offer with the flash sideways, in which Sayid's dead wife Nadia still lived. This juxtaposition too turned out to link unconnected situations.
- For fan theories about these unanswered questions, see: Flash sideways world/Theories