There are two main components to the Annunciation: the message itself, and the response of the Virgin. The message fulfills God’s promise to send a Redeemer (Genesis 3:15): “I will put enmity between you and the woman, between your seed and her seed; he shall crush your head, and you shall lie in wait for his heel.” The Fathers of the Church understand “her seed” to refer to Christ. The prophets hinted at His coming, which they saw dimly, but the Archangel Gabriel now proclaims that the promise is about to be fulfilled.
https://oca.org/saints/lives/2018/03/25/100884-the-annunciation-of-our-most-holy-lady-the-theotokos-and-ever-vi