The image is a simple one, and in the East one well knows, where the roads are comparatively few, and where they do exist are often in a bad state, when a sovereign is about to visit any part of his dominions, or still more if the march of an army has to be arranged for, the roads require considerable preparation.
There was a Jewish legend that this special pioneering work in the desert was done by the pillar of cloud and fire, which brought low the mountains and filled the valleys before the Israelitic march. John's special work was to prepare the way for the advent of a Messiah very different to the one the people looked for - to prepare his way by a spiritual reformation in the heart, the mind, and the character.
Here is the fulfilling of the Scriptures, Isa 40:3, in the ministry of John. When way is made for the gospel into the heart, by taking down high thoughts, and bringing them into obedience to Christ, by levelling the soul, and removing all that hinders us in the way of Christ and his grace, then preparation is made to welcome the salvation of God.
For instance, the mountains that must be levelled may be referred to the pride of the Pharisees; the valleys to filled up, to the moral and religious indifference of such as the Sadducees; the crooked places to be made straight, to the frauds and lying excuses of the publicans; and lastly, the rough places, to the sinful habits found in all, even the best.
[Excerpts from the Pulpit Commentary & Matthew Henry's Concise Commentary]