"Rejoice, you who are full of grace"
The Holy Spirit prepared Mary by his grace. It was fitting that the mother of him in whom "the whole fullness of deity dwells bodily" should herself be "full of grace." She was, by sheer grace, conceived without sin as the most humble of creatures, the most capable of welcoming the inexpressible gift of the Almighty. It was quite correct for the angel Gabriel to greet her as the "Daughter of Zion": "Rejoice." It is the thanksgiving of the whole People of God, and thus of the Church, which Mary in her canticle lifts up to the Father in the Holy Spirit while carrying within her the eternal Son.
Source: Catechism of the Catholic Church (Vatican.va)