What does Luke 3:18 mean?

Before John the Baptist was born, an angel predicted he would be a herald of the Messiah (Luke 1:16–17). As soon as he was born, his family began to anticipate the beginning of his ministry (Luke 1:76). John spent most of his life in solitude (Luke 1:80), developing an interesting lifestyle (Matthew 3:4). When he began to preach, his message was a call for repentance from sin (Luke 3:3–6). This corresponded to prophecies about a herald who would precede Messiah (Malachi 3:1; 4:5–6).

John's message involved exhortations: strong encouragements and passionate advice. He was certainly not afraid to speak his mind (Luke 3:3–9). Beyond what's recorded explicitly in Scripture, John's ministry was likewise bold and outspoken. Unfortunately, John's honesty was not always appreciated. Among the offended is the local ruler, Herod (Luke 3:1). John will openly condemn Herod's relationship with his brothers' wife, calling it illegitimate and against Jewish law (Luke 3:19–20; Matthew 14:1–4). Speaking truth to power, in John's case, leads to prison and eventually to execution (Matthew 14:5–12).