On Ajira 316, Jack reads Locke's suicide note. As he finishes reading, the plane shakes. Hurley asks Caesar to buckle his seat belts as the other passengers look on, apprehensive. A white flash of light slowly builds and Jack, Kate, Hurley, and Sayid are flashed to the 1970's.
"Through the Window" relates to the phrase which Eloise said earlier on in the episode about them returning to the Island in a window in time.