What the prayer says
The Hail Mary begins by addressing Mary as one who received God's grace and was blessed among women. It then names Jesus as the blessed fruit of her womb. These ideas come from the events surrounding Jesus' birth, when the angel Gabriel visited Mary in Nazareth and when Elizabeth welcomed her (Luke 1:28, Luke 1:42).
The prayer then calls Mary the mother of God and asks her to pray for sinners now and at the hour of death. Catholics use this prayer privately, during the Rosary, and in gathered worship. For them, it is not meant to replace prayer to God. It is a request for Mary's prayers, much as Christians might ask a trusted friend in church to pray for them.
And she spake out with a loud voice, and said, Blessed art thou among women, and blessed is the fruit of thy womb.
Why Mary is called the mother of God
That title can sound surprising at first. Catholics do not mean that Mary existed before God or created God. They mean that the child she bore, Jesus, is truly God the Son who became human. Elizabeth calls Mary the mother of her Lord, and John teaches that the eternal Word became flesh (Luke 1:43, John 1:14).
The title protects something central about Jesus. He is not merely a good teacher or a specially blessed human being. He is the Lord who entered our world, took on human life, and was born to Mary.
And the Word was made flesh, and dwelt among us, (and we beheld his glory, the glory as of the only begotten of the Father,) full of grace and truth.
Why Catholics ask Mary to pray
Catholics believe believers in Christ remain alive to God after death, and they understand the Hail Mary as asking Mary to join them in prayer. Scripture encourages Christians to pray for one another, and James describes the prayer of a righteous person as powerful (James 5:16). At the wedding in Cana, Mary also brings a need to Jesus, and he acts (John 2:3-5).
Catholics give Mary honour because God chose her for a unique part in the coming of Christ. Yet Christian worship belongs to God alone. Mary herself points away from herself and towards the Lord, rejoicing in God her Saviour (Luke 1:46-47).
Why some Christians do not pray it
Many Protestant and other Christians gladly honour Mary as Jesus' mother, but do not ask her to pray for them. They believe Christians should direct every prayer straight to God through Jesus. The New Testament says that Jesus is the one mediator between God and humanity, and it invites believers to come boldly to God for mercy and help (1 Timothy 2:5, Hebrews 4:16).
This is a real difference among Christians, but it need not become a reason for contempt. The shared centre is Jesus: Mary bore him, loved him, and trusted God with her life. Whether or not you use the Hail Mary, you can read Luke 1 slowly and let Mary's response shape your own, receiving God's word with humble trust (Luke 1:38).
For there is one God, and one mediator between God and men, the man Christ Jesus;