What does Luke 3:4 mean?

An angel prophesied the birth of John the Baptist, saying he would serve to turn many in Israel towards the Messiah (Luke 1:16–17). This corresponds to prophecies such as those found in Malachi 3:1 and Malachi 4:5–6. John's preaching ministry did just that: he told people about the coming of Christ and directed others to follow Jesus when He began His public ministry (John 1:26–34; 3:28–30).

Here, Luke quotes from Isaiah 40:3–5 to further prove that John the Baptist fulfills prophecy. Scripture not only predicted details such as the place of Messiah's birth (Micah 5:2) and the nature of His death (Isaiah 53:12). It also spoke of a messenger who would announce the Savior's arrival (Luke 3:3). John has spent most of his life living in the desert (Luke 1:80). When he began to preach, his message was precisely what both prophecy and his own parents anticipated (Luke 1:76–77).