"And God said, Let there be light: and there was light. And God saw the light, that it was good" (Genesis 1:3-4). When we see light/dark imagery, we immediately know what's up: good versus evil. Or bad, or whatever you want to call it. That's, like, Symbols 101. But there are a lot of variations on the good/evil dichotomy. For instance, light/dark can also be wisdom/ignorance, or clarity/confusion.
And this is precisely how Augustine likes to use the light/dark motif: as an opposition between knowing and not-knowing. Actually, the image he really likes to use is the one of light bursting through the darkness. For Augustine, things aren't just light/good and dark/bad. Light illuminates things that were, before, in darkness. For example, he says:
They come to you and their darkness grows bright when they accept the light by which all who accept it are empowered to become the children of God. (VIII.4.2)
Light breaking through the dark—check.
He also utilizes this symbol with more subtlety:
You alone are the life which never dies and the wisdom that needs no light besides itself, but illumines all who need to be enlightened. (VII.6.1)
In this passage, wisdom equals light, but it does so because wisdom is the illumination of ignorance. Which means that wisdom can't exist without ignorance. After all, if you were into that physics stuff, you might argue that light is just the absence of darkness, or darkness the absence of light. The point is, you need both in order for the metaphor to work.
"Wisdom" in this case refers less to general knowledge than it does to knowledge about God. But, as you can imagine, knowledge about God leads to a lot of other positive things as well, and dispels a lot of negative things. Knowing God has a ripple effect on the rest of your life:
How far the first gleams of your light have illumined me and how dense my darkness still remains and must remain, until my weakness is swallowed up in your strength. (XI.2.1)
Man, we are getting all sorts of positive/negative, light/dark pairs here. Not only do we have wisdom and ignorance, but also strength and weakness, and salvation and… whatever the opposite of salvation is. See, once Augustine has established the light/dark contrast, we can extrapolate almost any opposition from that original dichotomy.