When referring to the title "Co-Redemptrix", we must remember, the prefix "co" does not mean equal, but comes from the Latin word, "cum", which means "with". The title of Coredemptrix applied to the Mother of Jesus never places Mary on a level of equality with Jesus Christ, the divine Lord of all, in the saving process of humanity's redemption.
It denotes Mary's singular and unique sharing with her Son in the saving work of redemption for the human family. The Mother of Jesus participates in the redemptive work of her Savior Son, who alone could reconcile humanity with the Father in his glorious divinity and humanity. Jesus Christ, true God and true man, redeems the human family, as the God-man. Mary, who is completely subordinate and dependent to her redeeming Son even for her own human redemption, "my spirit rejoices in God my savior" (Luke 1:47), participates in the redemptive act of her Son as his exalted human mother. Because of her intimate and unparalleled sharing in the mysterious work with the divine Redeemer, Mary, human Mother of the Redeemer, has merited the Church title, "Coredemptrix" which literally means, "with the Redeemer".
See also: Catholic.net (This link will take you outside my web site.)