A rattlesnake gives a warning before it strikes. The symbol is read as a clear signal of danger or anger that asks to be heard in time.
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.