Alright, well, remember back to when everyone was trying to figure out what the hell was happening on that island, and there were all sorts of fan theories floating around? One of them was that they were actually all dead, and that the island was a form of purgatory. The creators themselves came out at one point and said that that particular theory was false, and the show went on.
But, in the final season, instead of the flashbacks, or the flashforwards, you had sideways flashes into what looked like a parallel universe, where all the characters lived different lives, and none of them knew each other. The big reveal in the final episode was that this parallel universe actually /was/ purgatory - they /were/ dead, and slowly gathering together again so that they could move on.
The fact that they all died at different times (some of them on the island, some of them surviving to live full lives between their time on the island and when they met again) apparently didn't matter, because their time on the island was so important to each of them that they all felt connected to each other - and, also because time works differently in the afterlife.
So, in the final moments set in the parallel universe (which was purgatory) you had everyone gathered, with Ben sitting outside because he didn't think he deserved to join them, and then the actual final scene set on the island was of Jack dying.
That's that part of it, anyway. Any of the actual reveals about what the hell the island itself actually was went right by me. I kind of stopped watching for a while, but I still ended up seeing the final episode.