For you were once darkness, but now you are light in the Lord. Before a person is saved, he or she is in darkness. But when a person becomes a Christian, he or she is made light in the Lord and is a child of light. Basically, light has to do with anything and everything related to God, Jesus, and things that are good and right. And so the opposite is true for the things of darkness.
Darkness is anything and everything related to the devil and things that are bad and wrong.
Verse 10 is a very important piece of this scripture. It instructs us to find out what pleases the Lord. Of course, the very best way to do this is to read the Bible. It is God's Word, and the more we learn from the Bible the more we will find out what pleases God. Once we know what pleases God, then we know those very things are of the light.