A blaring horn is traditionally read as a warning or a demand for attention. It may stand for someone trying to get through to you, or for your own impatience.
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.