I am the handmaid of the Lord; let it be done unto me according to thy word.
The words of the angel, words of comfort to a young girl who didn't know what to make of both his greeting and very appearance: "Do not be afraid."
He announced to Mary her role in salvation history, and with great compassion as well as great hope; it is said that all of Heaven held their breath in anticipation, awaiting her response. She had free will, and could have refused such a gift. But through Mary's yes, through her unconditional fiat, she brought salvation into the world in the person of her Son.
Mary's "yes" overturned Eve's "no" and effected the beginning of the reconciliation of man to God. Her complete and willful obedience was in answer to Eve's complete and willful disobedience.