The Star of David is named after King David of ancient Israel; sometimes it is called the Seal of Solomon after his son, King Solomon. It has been a generally recognized symbol of Judaism and Jewish identity since the Middle Ages, alongside the more ancient symbol of the menorah. Evidently, Judaism adopted the Star of David during the Medieval Period.
Judaism understands the Star/Shield of David as equating with the number seven, that is, the six points plus the center. The number seven has religious significance in Judaism, e.g., the six days of Creation plus the seventh day of rest, the six working days in the week plus Shabbat, the Seven Spirits of God, as well as the Menorah in the ancient Temple, whose seven oil lamps rest on three stems branching from each side of a central pole.
Kabbalistically, the Star/Shield of David symbolizes the six directions of space plus the center: Up, Down, East, West, South, North, and Center.
Non-Jewish Kabbalah (also called Christian or Hermetic Kabbalah) interprets the Star of David to mean the divine union of male and female energy, where the male is represented by the upper triangle and the female by the lower one.
Information source: Wikipedia