In Him was life, and the life was the Light of men. The Light shines in the darkness, and the darkness did not comprehend it.
-John 1:4-5 (NASB)
Here, John continues on with more abstract descriptions of the Word: He is the source of a life that brings light to all of humanity. The same living Word who was in the beginning, who was everything's first cause, continues providing life, light, and goodness to all who see Him.
In verse 5, we see our first indication of an adversary. Up until now, the Word has been unopposed, as well you might imagine. After all, think about what a potential opponent would be facing: A being that was in the beginning with God, that is one with God Himself, and that has brought all things into existence. It's not exactly taking candy from a baby to go up against this guy.
Nonetheless, this divinely powerful Light does have an opponent: The darkness. The word that the NASB renders "comprehend" is actually κατέλαβεν, and "comprehend" is only one possible meaning. Other senses of the word include "take hold of," "arrest," "capture," and "perceive." It comes from two words meaning "to aggressively take" and "down." Literally, the darkness cannot take the light down.
So the darkness could signify ignorance here: The Light was shining in the darkness, but the darkness failed to understand it.
Or the darkness could signify opposition: Despite its best efforts, the darkness couldn't stop the Light from overcoming it.
Regardless of which sense is correct here (and the answer might be that both are!), the Light stands in direct opposition to the darkness. It contends against darkness, evil, ignorance, and general sneakiness. This is a common theme in St. John's writings -- his first epistle reads, "This is the message we have heard from Him and announce to you, that God is Light, and in Him there is no darkness at all" (I John 1:5).
The stage is set, the actors are in position, and the lights have come up on center stage, overwhelming and driving out the darkness. (The darkness isn't completely gone, though -- he's hiding in the wings, preparing for his next appearance.) Now, at last, we're ready for the narrator to come on the scene...