In an alternate timeline the Apocalypse was never averted and the forces of Hell devastated the Earth.
History
Sometime after 2009, with the Winchester brothers parting on their separate ways, Sam said “Yes” to Lucifer, allowing the archangel to continue the Apocalypse. Dean however refused to fulfill his role as the vessel of Michael. With the angels unable to fight Lucifer, the archangel unleashed destruction upon the Earth and began the extermination of humanity.
Many humans became infected with the Croatoan Virus, entire cities were wiped out and humanity was nearing extinction. Sarah Palin became President of the United States and at some point sports were banned to stop the spread of the virus (which has spread worldwide).
Sometime before 2014, Dean Winchester became the leader of Camp Chitaqua, a group consisting of humans who have yet to be infected by the virus. Members included Castiel (who became human), Chuck Shurley (in charge of supplies), Risa (lieutenant), Jane, and formerly Bobby Singer. At some point, Bobby was shot to death in his house for unknown reasons.
In 2014, Dean from 2009 is sent into this timeline by Zachariah who wants to show him what will happen if he doesn’t consent to being Michael’s vessel. Here he learned of his future, that Sam supposedly died in Detroit years prior and that Future Dean tried to contact the angels, but at that point it was too late, the Host of Heaven had left. Future Dean planed a final attack on Lucifer, since he had finally located the Colt. Although everyone else was to act as bait, no one complained. The attack however was a failure and Future Dean was killed by Lucifer, who was possessing Sam.
Before being sent back to his time, Past Dean talks to Lucifer who informs him that whatever path he takes, their ending will always be the same.
Aftermath
Despite Zachariah's intentions to get Dean to say "Yes", the journey only convinces Dean to reunite with his brother to avoid this future, which they successfully do.