A mission or a missionary is read as a calling carried into unfamiliar territory. Dream books link it to conviction and service, and also to the question of how much you push your views onto others.
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.