Hail Mary is a Christian prayer to Mary, the mother of Jesus. Most of the prayer comes from the Gospel of Luke. Some things were also added to the prayer during the 13th century (the 1200s). When a person says a Hail Mary, they ask Mary to pray for them.
In Roman Catholicism, the Hail Mary is the most prevalent prayer in the Rosary. The Eastern Orthodox and Oriental Orthodox also use the prayer. So do many other groups within Christianity, including Anglicans, Independent Catholics, and Old Catholics. Some Protestant denominations also use the Hail Mary.