What does Luke 1:16 mean?

The Old Testament predicted a prophet would arise to herald the coming of the Messiah (Isaiah 40:3; Malachi 3:1). This prophecy will be fulfilled in the life of John the Baptist. An angel has just predicted John's birth to Zechariah (Luke 1:11–15), an aging, childless priest (Luke 1:5–7). The angel has noted that John will be called "great" for his spiritual strength (Luke 7:24–28). Before he is even born, the child will be sensitive to the influence of the Holy Spirit (Luke 1:43–45; 80).

Here, the angel notes that John's call for repentance (Luke 3:2–3) will be effective. Before Jesus begins His earthly ministry, John's preaching will "make his paths straight," introducing ideas which will make others more receptive to the preaching of Christ (Mark 1:4). Even after Jesus' crucifixion and resurrection, John's message will be a point of reference for evangelists (Acts 13:24–25; 19:4). However, his message will not be the final word in God's plan for mankind's salvation—this is a point John himself will fully understand (John 3:28–30).