Winston believes that the dream is referencing a time and a place where Big Brother's dark and oppressive oversight will no longer exist. Winston, and the reader, both believe it will be a promised land of sorts or a revolution in which all of Big Brother's manipulation will be exposed for all of the world to see. A light will shine down and overcome evil. It's a familiar literary motif.
Unfortunately, Winston has it all wrong. The dream is foreshadowing Winston's torture at the hands of O'Brien. That entire sequence will occur in a place where there is no darkness. . . because a light is always shining down on Winston. It's part of his torture, and it helps break him back down into a completely compliant party member.