The idea is simple. Mary, was born in a state of grace without original sin, thus freeing her to respond to gods call with a pure heart not bound by the selfish nature of original sin. The angel Gabriel comes, and announces the news of her role in gods salvation, to which Mary responds “let it be done to me according to your word”.
I like to contrast Mary to another figure from the Scriptures named Sarah. Sarah was the wife of Abraham. God had promised Abraham a child, a great nation of descendants to follow, but Abraham, and Sarah, were now getting very old, and no child had been born yet. Then, a “visitation”, three strangers come to Abraham, and announce that Sarah will have a child. Sarah laughs in response, saying to herself “After I am grown old, and my lord is an old man, shall I give myself to pleasure?”.
Now who in Sarah’s shoes would have thought any differently? And who in Mary’s shoes would have responded in such faith? We are looking at two “miraculous births”, and two biblical figures who respond quite differently to the word of the Lord. And in this difference can be seen the Grace that God bestowed upon Mary to respond the way she did to the promise of God.