Catholics believe that when you "offer it up," when you patiently endure suffering for the sake of the Gospel, or for the sake of what is right, or in order to gain mastery over your flesh and its desires, then merit comes from that which God then turns around and pours out upon the Body, for it's own strengthening and edification. This is not to say that the merit from Christ's work on the Cross was not enough. This is not to say that Jesus needed anyone's help in order to save mankind. This simply means that God chooses to involve human beings in his work, and that Mary, being a sinless human being and the one who gave the Son the very flesh that he nailed to the Cross for our salvation, was able to stand alongside the Redeemer and cooperate with his saving plan in a way that no other human being was able to do.
For that reason, a particular title is reserved for her, and that title is Coredemptrix. None of what I have said here should be misconstrued as meaning that Jesus is not the only Savior, or that Jesus needed Mary's help, or that the merits of the Cross aren't enough, or anything of that sort. Salvation comes from Christ and Him alone. Mary would be just a typical girl without Him.
For more information on Mary as the Coredemptrix, I suggest the following articles:
- Mary Coredemptrix: A Response to Seven Common Objections
- Unscriptural Marian Doctrine?
- Mary's Cooperation in Redemption: Parts One and Two
- Mary's Role in Our Salvation
- A Biblical and Theological Primer on Mary as Mediatrix (and Coredemptrix)