What does Luke 1:73 mean?

Israel's most treasured prophecy was the promise to send a Messiah—a Savior—to defeat evil and sin once and for all (Isaiah 9:6–7). This was the ultimate end of the "oath" mentioned here, which God established with patriarchs like Abraham (Genesis 22:16–18). Each generation since had lived and died without seeing that promise entirely fulfilled (Hebrews 11:39–40), though they did see God's work in moving towards those ends (Hebrews 12:1; Exodus 6:8; 1 Kings 1:48).

Zechariah is a priest (Luke 1:5) whose wife, Elizabeth, has just given birth to a son (Luke 1:57). This child is the fulfillment of prophecies about a herald (Isaiah 40:3; Malachi 3:1; 4:6) who would prepare people for the Messiah (Luke 1:13–17). This child will soon be known as John the Baptist (Luke 3:2–3). In another confirmation of God's promises, a young virgin has learned she will give birth to this Savior (Luke 1:31–33). Zechariah's song of praise (Luke 1:67–68) recognizes these events as further proof that all God said would happen truly will occur.