"What happened was this: I got an image in my head that never..." — Robert Penn Warren quote meaning

What happened was this: I got an image in my head that never got out. We see a great many things and can remember a great many things, but that is different. We get very few of the true images in our heads of the kind I am talking about, the kind that become more and more vivid for us as if the passage of the years did not obscure their reality but, year by year, drew off another veil to expose a meaning which we had only dimly surmised at first. Very probably the last veil will not be removed, for there are not enough years, but the brightness of the image increases and our conviction increases that the brightness is meaning, or the legend of meaning, and without the image our lives would be nothing except an old piece of film rolled on a spool and thrown into a desk drawer among the unanswered letters.