In the last season, instead of flashbacks, they showed these weird "flash sideways", where the same characters were interacting but in totally different ways, like they were in a parallel universe. The plane never crashed in these flash sideways, but characters that were connected on the island would end up being connected in this parallel world.

At the end, you learn that these "flash sideways" were actually post-death experiences in some post-death world. But it didn't negate anything that happened to the characters while they were living on the island. They all crashed on the island, traveled through time, and all that other fun stuff. When they all eventually died, whenever that ended up being, they were in the "post-death" world where all the flash-sideways took place.

Anyway, the show became overly convoluted in the last few seasons. I think the writers wanted to add some shock value in the last season with the "flash sideways" but it just made people confused. I don't really blame anyone for not liking how it ended. I stuck with it and enjoyed it but realized a lot of it was the writers just making stuff up as they went along that didn't all resolve by the end of the series.

Regardless, the pilot and season 1 are some of the best storytelling I've ever seen in a TV show.