How to understand "He prospers and I declines"

He will prosper, and I will decline, saying, "He who comes after me has become before me. Because he was before me. "Many people will also rejoice at the birth of John. He is great before the Lord; he drinks neither wine nor strong drink; he is filled with the Holy Spirit from his mother's womb. He will turn many Israelites back to the Lord their God. He will walk before the Lord with Elijah's mind and power, and turn his father's heart to his children and his rebellious heart to the wisdom of the righteous. And prepare a people for the Lord. He is not light, but a witness of light. John, the witness of Jesus, cried out, saying, I said, He who comes after me has become before me. Because he was before me. I baptize you with water so that you may repent. But the one who comes after me is more powerful than me, and I am not worthy to carry his shoes. He will baptize you with the holy spirit and fire. "