What does Luke 1:55 mean?

When Mary heard she was going to bear the Messiah (Luke 1:31–33), she knew it would fulfill one of Israel's most cherished prophecies. God had made promises to patriarchs like Abraham (Genesis 17:9), Isaac (Genesis 28:13), and Jacob (Genesis 35:10–12). Later prophets had predicted the arrival of a Promised One who would finally set all things right and rescue Israel (Isaiah 9:6–7; 2 Samuel 7:12–16). Each generation since Abraham had seen progressively more of the Lord's promises come true. God's declaration that Jesus Christ will be born validates His promise not to forget the people of Israel (Luke 1:54; Psalm 132:11).

This ends Mary's song of praise, which is often called "The Magnificat." She speaks these words (Luke 1:46–54) as she celebrates with her relative, Elizabeth (Luke 1:39–42). Elizabeth is six months pregnant (Luke 1:37) with a child who will become known as John the Baptist (Luke 3:2–3). Mary, it seems, will stay with Elizabeth until she gives birth before returning to her home (Luke 1:56). There, she will await her marriage to Joseph (Luke 1:26–27). Scripture does not say if Joseph, at this time, was aware of Mary's condition, though he will learn about it soon (Matthew 1:18–21).