A roll of film stands for memories waiting to be developed. Dream books read it as experiences you have not yet made sense of, kept in the dark until the time is right.
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.