Our poor speaker has seen a vision of his dead wife, who has appeared to him from beyond the grave. But it's not as spooky as you might guess. Instead of chatting about how petrified he is, he describes his wife using a series of similes comparing her to figures from religion and mythology.
Then he delivers the goods—she's both the earthly woman he loved, and an ethereal version of herself, free from sin and imperfection, which is not so bad, all things considered. Then, as the vision leans down to give the speaker a smooch, he wakes up and realizes it was all a dream. Bummer.