It's probably foreshadowing something in your life, good or bad.
A ghost, in dreams, indicate that you may be disconnected from life and/or society. A ghost may also represent an aspect of yourself that you may fear, this can involve a painful memory or guilt or some repressed thoughts. It can also mean you're afraid of death or dying. The ghost may even represent the angel of death.
If you do or have lucid dreams try and see if the ghost is trying to tell you anything else. Remember, don't take dreams TOO seriously.