"The icon of the Sign is among the most revered icons of the Mother of God in the Orthodox Church.  Mary is in praying posture with Christ on her breast.  The Old Testament prophecies of the Divine Incarnation culminate in the clear and distinct prophecy of Isaiah:

Therefore the Lord himself shall give you a sign: behold a virgin shall conceive in her womb and shall bring forth a son and thou shalt call him Emmanuel (Isaiah 7:14)

The image of the Mother of God with the Child Emmanuel in her bosom is this very Sign.  The Sign is the image of the Divine Incarnation, of the revelation of the Second Person of the Holy Trinity, the manifestation of the Son of God through the human nature received from the Mother of God.  Like Isaiah's prophecy itself, the icon of the Sign is a manifestation of the prophetic predictions of the Divine Incarnation."

(An extract from The Meaning of Icons by Ouspensky and Lossky)