A turtleneck covers the throat, so dream books read it as guardedness about what you say or feel. It can also stand for quiet sophistication and a composed, private self.
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.