Mary is the mother of Jesus,the saviour of the world
Mary, by agreeing to bring Jesus into the world, became a "cause" of our Salvation.
In doing so, she mediated between Heaven and earth. Therefore, the Church gives Mary certain titles such as Mediatrix of all Grace and Co-Redemptrix.
Again, when we hear titles such as these, we must always remember that these titles are in no way equating Mary to Jesus. To the contrary, they are meant to be understood in relationship to Jesus Christ. Mary is what she is by grace and by virtue of being "in Christ." Since Jesus Christ is the source of all grace and Mary brought Jesus into the world, Mary is the Mediatrix of grace. Since she cooperated with God's redemptive plan, she is Co-Redemptrix.