The mockingbird sings every song but its own, so the dream reads it as finding your own voice among borrowed ones. Since the famous novel, it also stands for innocence that should not be harmed.
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.