This is the very beginning of the Book of Consolation of the prophet Isaiah, which foretells Israel's deliverance from the Babylonian exile, and besides, as always in the prophetic books, much else. And the first image he uses is so understandable and vivid that later another prophet, John the Baptist, will quote it in his preaching. Indeed, imagine: God is heading through the hilly desert separating the Promised Land and Babylon in order to accomplish a new Exodus, and He sends ahead His herald so that people will prepare a road worthy of the King of kings.
It is interesting how these words were refracted in people's consciousness later: the colon moved to another place, after the words "in the wilderness," and the meaning changed. "A voice crying in the wilderness" means someone is shouting and no one is listening. That was not so with either Isaiah or John. Why, then, did the emphasis shift this way? Perhaps because in Christian times these words naturally began to be interpreted in a spiritual sense: the Lord comes to each of us to save us from sin and death, but in order for Him to enter our heart, repentance and the correction of our life ("preparing the way") are necessary for us. And this is where our "deafness" shows itself, and everyone who calls us to repentance turns out to be in emptiness...