A film negative shows the light reversed, so it is read as seeing the hidden side of a situation. Tradition links it to what is still undeveloped, and to opposites that belong together.
Has this come up in your dreams before? What was going on in your life then?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.