The Church’s teaching makes a clear distinction between the Mother and the Son in the work of salvation, explaining the Blessed Virgin’s subordination, as co-operator, to the one Redeemer.
Moreover, when the Apostle Paul says: “For we are God’s fellow workers” (1 Cor 3:9), he maintains the real possibility for man to co-operate with God.
[...] However, applied to Mary, the term “co-operator” acquires a specific meaning. The collaboration of Christians in salvation takes place after the Calvary event, whose fruits they endeavor to spread by prayer and sacrifice. Mary, instead, co-operated during the event itself and in the role of mother; thus her co-operation embraces the whole of Christ’s saving work. She alone was associated in this way with the redemptive sacrifice that merited the salvation of all mankind. In union with Christ and in submission to him, she collaborated in obtaining the grace of salvation for all humanity.
The Blessed Virgin’s role as co-operator has its source in her divine motherhood. By giving birth to the One who was destined to achieve man's redemption, by nourishing him, presenting him in the temple and suffering with him as he died on the Cross, “in a wholly singular way she co-operated ... in the work of the Saviour'' (Lumen gentium, n. 61). Although God’s call to co-operate in the work of salvation concerns every human being, the participation of the Saviour's Mother in humanity’s Redemption is a unique and unrepeatable fact.
[...] The Second Vatican Council moreover presents Mary not only as “Mother of the divine Redeemer”, but also “in a singular way [as] the generous associate”, who “co-operated by her obedience, faith, hope and burning charity in the work of the Saviour”. The Council also recalls that the sublime fruit of this cooperation is her universal motherhood: “For this reason she is a mother to us in the order of grace”.
We can therefore turn to the Blessed Virgin, trustfully imploring her aid in the awareness of the singular role entrusted to her by God, the role of co-operator in the Redemption, which she exercised throughout her life and in a special way at the foot of the Cross.
Pope Saint John Paul II, General Audience of April 9, 1997