What does Luke 1:29 mean?

Appearances from angels usually evoke instinctive fear or unease in those they visit (Luke 1:13; 2:10; Matthew 28:4; Acts 10:3–4; Numbers 22:31; Judges 6:22–23). This is partly due to their powerful, spiritual nature; Scripture does not include western art's serene, infantile versions of angelic messengers. Anxious reactions are also based on reputation: angels are not always sent in peace (2 Kings 19:35; 2 Samuel 24:16). Assurances not to be afraid are common, and Gabriel (Luke 1:26) will make just such a remark to Mary in the next verse (Luke 1:30).

Gabriel's first statement to Mary was a greeting indicating that she had been esteemed by God (Luke 1:28). Despite translation errors that suggest otherwise, Mary was not "full of grace," or a "source of grace," but one who was given special blessing. Mary's response here further goes to support this truth. Her reaction is to wonder—quite naturally—what exactly it means that she's been "favored," and that God is with her. God's calling can involve drastic steps (Genesis 12:1; Judges 6:11–12; Luke 5:27–28). In this moment, she is probably apprehensive about what she will hear next.