Someone in the comments box recently asked whether the Catholic Church teaches that there are "two mediators" - Christ and Mary His Mother.
In a relative sense, anyone who prays for another is mediating petitions on behalf of the other party. However, in an absolute sense, "there is one mediator between God and men, the man Christ Jesus" (1 Tim 2:5).
Christ is fully God and fully man and thus He is the only absolute mediator between God and humanity. The Blessed Virgin Mary is human and not divine. Since Christ is the only mediator between God and humans, He is therefore the mediator between God and Mary.
Mary's mediation, like that of all Christians, participates in the priesthood of Christ or the general priesthood of the believing faithful. Mary's human prayers are the most efficacious because 1) she is the Mother of God; 2) Christ honors His mother in accordance with the Ten Commandments; and 3) since she provided the flesh and blood to the Logos, her contribution in the economy of salvation is unique.
If Christ is the Head of the Body of Christ, Mary is the neck.
I hope that helps.
- Taylor Marshall