from lost wiki
What is the flash-sideways reality? As Christian Shephard explained at the end, the alternate timeline, in which Oceanic flight 815 lands safely in Los Angeles, is a purgatory, a collective dream, mutually created by the souls of the castaways (and a few close outsiders, like Penny), a way for them to rediscover each other before moving on to the next spiritual plane. It's also a place for the characters, all of whom have heavy baggage (and not the kind you pick up at the carousel), to let go. (Which is why those characters not ready to let go, like Ben or Michael or Ana Lucia, didn't end up in the church at the end.) Does that mean they all died? (The shots of the empty plane wreckage over the closing credits has led some viewers to infer that no one survived the initial crash.) Well, they did all die, but not at the same time. Everything that occurred on the island, including the deaths of Boone, Shannon, Juliet, Sayid, Jin, Sun and Jack, really did happen. But so did the escape of Kate, Sawyer, Claire, Miles, Frank and Richard on the Ajira jetliner; they died sometime later (hopefully, after making it to safety and living out their lives). Hurley and Ben stayed behind to run the island and also, presumably, lived out their lives, as did Rose and Bernard. In the collective dream limbo, however, time doesn't matter, said Christian, so the souls didn't meet up there until everyone was dead. (What about Desmond? Did he make it off the island? The finale seems to have glossed over that question.)
The result looked something like a high school reunion -- everyone dolled up for a party, sharing hugs and memories -- but it made for a nice twist on Jack's motto: "Live together, die alone." Well, turns out we don't even have to die alone.
As for where the souls were going next, into the golden light behind the door Christian opened, was that heaven? Maybe, but 'Lost' wasn't interested in any particular religion's cosmology. That was evident from the ecumenical nature of the stained-glass window, which included emblems from Judaism, Christianity, Islam, Buddhism, Taoism and Hinduism. There's an afterlife waiting for everyone, as long as they have faith and are willing to let go
I was very happy with the ending, lots of comments about not getting enough answers etc. The show has always been about the journey of the characters, i found it very fitting that the show ended with our main characters journey and redemption coming to an end.