How could you explain a dream in "The Rime of the Ancient Mariner"?
It depends on which dream you might be referring to. In Part Two, some of the crew dreamed that the spirits that plagued the ship becalming it and depriving it of rain were caused by the albatross seeking its revenge. Because of this dream, the dead bird was hung around the mariner's neck. In Part Five, the mariner was finally able to sleep after seven days. He dreamed the buckets on the deck were full of dew, then upon awakening, he discovered it rained. The poem says the mariner awoke at this point, but what follows seems dream-like with the dead sailors arising to man the ship and the wind moving it forward. The mariner says the men weren't ghosts but were blessed spirits. So, I don't think that part was a dream. Toward the end of Part Five, the ship pitches violently and throws him into a "swound" (he faints) and as he lay unconscious he heard two spirits speaking. The spirits identified him as the killer of the bird and announced he had penance to do. The spirits remark about how the ship is moving so quickly without any wind. Then the mariner awakes to normal weather and fair progress as the dead men stare at him with their eyes cursing him. The spirits then depart the sailors' bodies as blessed spirits. So much of the errieness occurs when he is supposedly awake rather than asleep.