John 1.6-8:
There was a man sent from God, whose name was John. (7) He came as a witness, to bear witness about the light, that all might believe through him. (8) He was not the light, but came to bear witness about the light. (ESV)
The apostle John, writing in the beginning of his gospel account, refers to John the Baptist in the verses above. John the Baptist was the forerunner who went before Jesus to announce the coming of Jesus as the Messiah. John was well respected and some people even speculated that John might himself be the Messiah. The verses above correct this idea and in so doing this gospel equates the Messiah and the Light.
In reality Jesus is the Light of the universe, and He is the source of all true enlightenment and understanding. We can therefore walk in the light and see the right path. No darkness, especially spiritual darkness and the forces of evil, can rise up against Jesus and successfully oppose Him. Darkness does not win when it encounters light. By John the apostle equating Jesus as the Light, and the Light as the Messiah, he is confirming that Jesus is God.