Mary's special cooperation in the Redemption

Predestined from eternity by that decree of divine providence which determined the incarnation of the Word to be the Mother of God, the Blessed Virgin was on this earth the virgin Mother of the Redeemer, and above all others and in a singular way the generous associate and humble handmaid of the Lord. 

She conceived, brought forth and nourished Christ. She presented Him to the Father in the temple, and was united with Him by compassion as He died on the Cross. In this singular way she cooperated by her obedience, faith, hope and burning charity in the work of the Saviour in giving back supernatural life to souls. Wherefore she is our mother in the order of grace.

Lumen Gentium Chapter VIII §61 

Vatican II Council