A mask is a symbol in both the waking and dream world that covers something. If you experience a mask in your dream, what is being covered? Who is doing the covering? Is it you? Perhaps you are withholding information or yourself from someone in your life, that person would be involved with you in your dream in some way. Even if you do not see them in your dream, chances are you will have a sense of who the mask is for in the dream.
Another possible latent meaning of the mask symbol in a dream is that you are having feelings or insecurities with how others perceive you, and prefer to wear a mask to keep that part of you undisclosed for the time being. Look for clues in your dream as to whether or not that is an effective tool for the situation the dream is working around. The feelings and people in your dream will be very revealing in terms of what specifically the mask is pointing your attention to.