What does a Sikhs red dot on there head mean?

It depends on the part of India you are referring to. In certain regions it means the woman is married, in others it is simply a body decoration. It is in between the eyebrows because this area is a major nerve center in the human body. It is also the sixth chakra, the third-eye chakra. It is the seat of wisdom and is related to clear insight. Also because i said so (: