A trade union stands for solidarity and the strength found in numbers. Dreaming of one is often read as a wish for backing, or for someone to stand beside you when you ask for what is fair.
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.