What does Luke 1:70 mean?

In the Old Testament, God made many promises about the Savior, Jesus Christ, including details about His birth. These included the fact that He would be virgin-born (Isaiah 7:14; Luke 1:34–35). The prophecies noted that He would be from the lineage of David (2 Samuel 7:12–16; Jeremiah 23:5) and noted the power He would wield (Isaiah 9:6–7). Other prophecies mentioned a herald to prepare the world for Jesus' message (Malachi 3:1; 4:6; Isaiah 40:3).

Zechariah, the man speaking the words of this passage, is the father of that herald (Luke 1:13, 57). His song of praise emphasizes how God's promises, as given through His prophets, have always come true. That the world has seen the fulfillment of prophecy and will soon see more (Luke 1:39–45), is reason to worship. That also comes with joy, as the role of this Messiah will be that of Savior (Luke 1:71).