The mark of the ash cross on the forehead is a reminder we are mortal as in "ashes to ashes dust to dust". Receiving an ash cross on the head is an act of humility and a recognition we were created by God and are mortals. It is done not in mourning but as a beginning of lent. Lent is a period of introspection and prayer as we all journey together the 40 days to Easter. It's not a sign to show we are holy, it is a sign to show solidarity with the faithful as we shall all journey together. The ashes are a recognition of human frailty not just in that we are mortal but that we are all sinners and in recognizing that we need to forgive each other and ourselves as well as ask God for forgiveness.