Ill break it down.
D
WARNING SPOILERS IN THIS POST.
The show is about a group of strangers that somehow survive a horrific plane crash and get trapped on a mysterious island. EVERYTHING THAT HAPPENS ON THE ISLAND HAPPENED. They were not dead...they were not in purgatory....they were not dreaming or imagining anything. EVERYTHING THAT HAPPENED ON THE ISLAND AND OFF THE ISLAND HAPPENED. They were hand picked by the island caretaker Jacob. Jacob has a un named brother referred to as the man in black....he can take the form of dead people. He also happens to be a "smoke monster". Smokey wants off the island more than anything....he's been there for thousands of years. The island basically is a balance of good and evil in the world and if he gets off, the scale tips in evils favor.
There were flash forward seasons and there were flashback seasons. There were time travel seasons.
Season six (the final season) was a FLASH SIDEWAYS. It was....for lack of a better term...a separate timeline that ran concurrent to the island time line. BOTH OF THOSE TIMELINES WERE HAPPENING AT THE SAME TIME.
Ill admit, season six gets tricky....but if all the cry babies would watch the finale again....it's does a fantastic job of explaining everything.
Season 6 is basically being told through Jack Shephards point of view. Jack assumes the role of Jacob on the island (briefly) and winds up suffering a fatal knife wound prior to killing smokey. He dies in the bamboo forest just after SEEING HIS FRIENDS FLY OVERHEAD IN AJIRA FLIGHT 316....
Meanwhile in the "sideways world" all the LOSTIES are bumping into one another....and they're getting glimpses of their life back on the island. Jacks dead father explains to Jack that THE TIME JACK SPENT ON THE ISLAND WAS THE MOST IMPORTANT TIME IN HIS LIFE. That all those people he was with were all meant to be together. This "sideways world" was basically a place that they created to re connect with one another so that they could all "move on" together.
In the sideways world, there is no time...it doesn't exist the way it does in life. Jack died on the island and Kate flys overhead in AJIRA flight 316 but Kate says to him " I've missed you so much" . So she has lived out the rest of her normal life. Hurley says to Ben, "You were a great number two"...Ben replies "And you were a great number 1" ( Hurley and Ben wound up as island caretakers after Jack died). Those two watched over the island for what may have been hundreds or thousands of years.
But here they all are together....because time doesn't exist the same in the after life.
YES....NOW THEY ARE ALL DEAD!!!!