Locke likens Charlie's struggles with his heroin addiction and his inner strength to fight it off as a moth breaking through its cocoon. Later, when trapped in a cave-in, Charlie sees a moth and remembers what Locke said, and in following it, successfully rescues Jack and himself.
The episode signifies both the beginning of the end of the Lostie's time on the island, the beginning of the end of the show itself (this is the halfway point of the six seasons), and also references Ben's quote in "Through the Looking Glass, Part 1", "If you make that call, Jack, it's the beginning of the end."
The survivors are "confirmed dead" by the cameras at the beginning of the episode. The flashbacks in this episode largely involve the reactions of Daniel, Charlotte, and Frank (and to an extent, Miles) to this confirmation).
Sayid's work after being rescued from the Island involved heading to Germany to track down a man who Elsa, his employee, referred to as "an economist."