The Incarnation means that the Second Person of the Blessed Trinity, the Son of God (the Divine Word), became man and came to live among us.
“And Mary said, ‘Behold, I am the handmaid of the Lord; let it be to me according to your word’” (Luke 1:38). At that very moment Jesus Christ, God’s own Son, became man in the Blessed Virgin Mary.
The Son is begotten from the Father, and the Holy Spirit proceeds from the Father and the Son. By an act of His almighty power, God the Son united His own divine nature to a true human nature, to a body and soul like ours. The two natures were united in one divine Person. Mary is the Mother of God because she is the Mother of God the Son, Who became man in her womb.
Mary gave her free consent to become the Mother of Jesus. At that moment, by the operation of the Holy Spirit, she conceived Jesus within her womb. The result was one divine Person with one divine and one human nature.