If I understand it right when a Catholic prays to Mary he is asking for Mary to pray with him. We can go directly to Jesus without speaking to Mary if we choose, so I would assume that: Mary has a special relationship with God that allows us to “tap into” God more efficiently—as though Mary is a special conduit to reach God more effectively than just praying to Christ alone. Is this correct?
When we pray to Mary and other saints we are asking for their intercessory prayers. For example when we say the *Hail Mary * we ask, “Holy Mary, Mother of God, pray for us sinners now and at the hour of our death.” This is similar to asking any Christian to pray for us but, since Mary and the saints are in the presence of God, their prayers are even more efficacious.
For more on this see Praying to the Saints.