The last of the great Old Testament prophets has made his way to the Jordan River, most likely in the southern region of the land. His name is John and he is sent by God. His purpose is to witness, to testify about the Light of the world, namely, Jesus Christ who is the Word of God (John 1:1).
When John enters the scene, he no doubt speaks of the Light as someone other than himself. He knows that he is not the Light but has made his way to the river to announce the coming of the Light of the world. He is only here to testify about the Light who is now come.