Addressing Our lady with this title, we praise her privilage to be chosen the Mother of God and Saviour, Jesus Christ.
Our Lady "was enriched by God with gifts appropriate to the role of Jesus mother". "Full of grace" was therefore salutation of Archangel Gabriel at the moment of the annunciation was - "full of grace". In fact, in order for Mary to be able to give the free assent of her faith to the announcement of her vocation, it was necessary that she be wholly borne by God's grace. Through the centuries the Church has become ever more aware that Mary, "full of grace" through God, was redeemed from the moment of her conception. That is what the dogma of the Immaculate Conception tells us as was proclaimed by Pope Pius IX in 1854: The most Blessed Virgin Mary was, from the first moment of her conception, by a singular grace and privilege of almighty God and by virtue of the merits of Jesus Christ, Savior of the human race, preserved immune from all stain of original sin. The "splendor of an entirely unique holiness" by which Mary is "enriched from the first instant of her conception" comes wholly from Christ: she is "redeemed, in a more exalted fashion, by reason of the merits of her Son". God the Father blessed Mary more than any other created person "in Christ with every spiritual blessing in the heavenly places" and chose her "in Christ before the foundation of the world, to be holy and blameless before him in love". The Fathers of the Eastern tradition call the Mother of God "the All-Holy" (Panagia), and celebrate her as "free from any stain of sin, as though fashioned by the Holy Spirit and formed as a new creature". By the grace of God Mary remained free of every personal sin her whole life long.
Let us pray that Our Lady conceive without sin will lead us to detest the sin! Let us call her every time are tempted: "O Mary, conceived without sin, pray for us who have recourse to thee"!
after Catechism of the Catholic Church-Vatican website
Picture is "Mary Immaculate" Cuzco School of Art