What does Genesis 1:4 mean?

This is the first of several times in the creation account where God will pronounce what He has just made as "good." He made light, and He approved of it. Pointedly, God did not call the darkness good. In Hebrew philosophy, "light" was the ultimate ideal, and a symbol of wisdom, goodness, and knowledge. There is powerful symbolism in God's choice to create light among the darkness of the universe.

This verse begins a pattern repeated for the rest of the passage. In each of the next days of creation, God will speak something into existence, see the effect it has, declare it good, and then the text will declare the number of the day.

Here, God is said to have separated darkness and light. The two would exist in the world separately from each other, with light being the dominant force. To the extent that light appears, darkness will always disappear. Darkness has no defense against light, since "darkness" does not really exist, in and of itself. It is simply the absence of light.