The quotation is from the ancient Greek version of Isaiah 40:3–5. It describes God bringing the Jews back from exile in Babylon. After they returned (Ezra 1–2). John shouted in the wilderness to prepare God’s people for the Lord’s coming. God’s salvation is portrayed as a new exodus, bringing deliverance like the first exodus from Egypt.
make straight paths for Him … the rough ways smooth: This image reflects the Middle Eastern custom of preparing a road for a king (compare Luke 19:36–38).