The "Star of David" or Magen David as it is known in Hebrew literally means the "Shield of David". According to the Kabbalah, the two triangles facing opposite directions represent the dichotomies inherent in man: good vs. evil, spiritual vs. physical, etc. The triangle that is pointing up symbolizes our good deeds, which are directed towards heaven and then activate a flow of goodness that comes back down into the world, which is symbolized by the triangle that is pointing down.