Chaire kecharitomene (Greek) is the salutation the Angel Gabriel gives to Mary in Luke 1:28. Ave gratia plena is how Saint Jerome translated that phrase into Latin.
Greek and Latin give the same meaning: Hail, full of grace.
Kecharitomene comes from the root word charis, meaning “grace.” Grace is a supernatural gift from God given to man. If Mary is full of grace, it is by the power and the will of God. The Angel does not give her the grace, he merely states a fact that she is full of grace; this is before she conceives in her womb the child Jesus, the Son of God.
“Full” means one is at one’s capacity. It does not mean that Mary is the source of grace or that she has all the grace in the universe, merely that she herself is filled with God’s grace.
Since the sin of Adam and Eve, no children or descendants of theirs had sanctifying grace, which makes one a child of God and able to enter heaven. Grace was lost by sin. So, how did Mary get it? She got it from God.
Jesus Christ redeemed the entire human race, including His human mother, Mary. He, being God, could also apply to her the benefits and fruits of His work of redemption backward in time so that Mary could receive grace at the moment of her conception in the womb of her mother, Saint Ann (grandmother of Jesus).
The doctrine of the Immaculate Conception is merely that God applied or gave to Mary the fruits of what her Son Jesus would do in her future by having Jesus give backwards in time from His Passion and Death on Good Friday. In other words, what Jesus did in Mary’s future, He gave her the application of in her past.
She was conceived without original sin so she could give her Son an untainted human nature. This gift came from her Son, who would become the Savior and Redeemer of mankind.
Mary did not know this had happened to her. She did not glow in the dark, and there was no sticker or label on her telling anyone about it. Only when the angel told her she was full of grace did she find out. Once she said yes to Gabriel, the Holy Spirit overshadowed her, and she conceived Christ in her womb without the assistance of any human father.
To be “full of grace” meant she was filled with the source and author of grace, God Himself.