The Annunciation is one of the seeds for the dogma of the Immaculate Conception. The Archangel Gabriel greets Our Blessed Lady as "Full of Grace." He gives her a new title and announces that Mary will be a mother to the Savior of the world; God-made-man.
At the Annunciation we see the fulfillment of the prophecy of Isaiah 7:14: "Behold a virgin shall conceive and bear a son, and shall call his name Emmanuel."
This is an extraordinary moment in salvation history for the very fact that God, the creator of the world, humbled himself and became man inside the perfect womb of Mary which He Himself had created. God did not need to use a mere creation in His mission of salvation, but He chose to do so. God enacted what the early Church Fathers called recirculation: As death entered the world through a man, a woman, and a tree; so it was fitting that our redemption came about by a man, a woman, and a tree. Thus, Christ chose His mother (who albeit a perfect creation is still a creation nevertheless) to actively participate in the redemption of the world.
Mary’s "yes" to the Archangel is a "yes" for all humanity. She mediates Christ to the world. St. Bernard said that the whole world waited to hear Mary’s "yes." Her "yes" was the most important "yes" in all of history. With it she gave consent for God to dwell within her. She became the tabernacle of the Word; the ark of the New Covenant! For Mary, the Annunciation is a type of anticipated Pentecost where the Holy Spirit, her spouse, overshadows her. The uncreated Immaculate Conception overshadows the created Immaculate Conception. The two become one.
Our Lady’s "yes" at the Annunciation is also a "yes" to Christ’s entire mission of redemption, including Calvary. She does not give a separate "yes" in the offering of her Son upon the Cross, but rather her "yes" is a universal fiat. She says, "Let it be done unto me according to thy word." Even if that means sacrificing her Son for the sake of the world and having a sword mystically pierce her heart as well. As Christ was crucified physically at Calvary, Mary was spiritually crucified with her Son. For this reason the early Church Fathers describe the Incarnation as redemption anticipated and begun. For this reason also, we hail Mary as co-redemptrix: the woman with the redeemer.
"[Mary] offered her virginal womb for the incarnation of God's word. At the annunciation Mary conceived the Son of God in the physical reality of his body and blood, thus anticipating within herself what to some degree happens sacramentally in every believer who receives, under the signs of bread and wine, the Lord's body and blood."
-John Paul II