The Third Eye chakra is about insight and visualization. When it is open, you have a good intuition. You may tend to fantasize.
If it is under-active, you're not very good at thinking for yourself, and you may tend to rely on authorities. You may be rigid in your thinking, relying on beliefs too much. You might even get confused easily.
If this chakra is over-active, you may live in a world of fantasy too much. In excessive cases halucinations are possible.