A contrail is the trace of a journey already passing, so it stands for departures, distance and things moving on. Dream books read watching one as thinking about someone far away.
When this appeared in your dream, what were you feeling?
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.