This episode deals with the physical and emotional act of leaving the Island.
This series finale is the literal embodiment of "Letting Go."
The characters reach a point of spiritual fulfillment where they choose to step through the final door and dissolve into the universe.
The "letting go" here is literal, as the freighter explodes with Jin still on board, forcing Sun to leave him behind. 4. Community: "Advanced Introduction to Finality " (S4E13) The finale of the controversial "gas-leak year" (Season 4).