What does Luke 3:5 mean?

This continues a quotation from Isaiah 40:3–5. Luke is explaining how John the Baptist fulfilled prophecy by proclaiming the arrival of the Messiah (Luke 3:1–4). Isaiah predicts a single voice in the wilderness, and John has lived most of his life in the desert (Luke 1:80). When he began to preach, his message was for Israel to repent and be ready to meet the Savior (Luke 1:76–77). John called on people to turn from sin, but also made it clear he was not, himself, the Promised One (John 1:26–34; 3:28–30).

When Isaiah wrote his words, they were presented to the people of Israel as a prediction that God would bring them back from exile. The secondary application is that of the lone herald speaking about the coming Messiah. This is further supported by other Old Testament prophecies that indicate a herald will precede Christ (Malachi 3:1; 4:5–6). John's preaching ministry will turn "rough places" into "level ways," by preparing others to hear and accept Jesus' message (John 19:1–5).