For the fruit of the light consists in all goodness, righteousness and truth.
Fruit of the light are much like fruit of the Spirit. The source of all good things is God. The fruit of the light can be thought of as the outcome of walking in the light with God.
Righteousness is something that we cannot achieve but yet we still strive for it. The point of the law of Moses was to cause us to realize that we were incapable of being righteous and we needed a savior to forgive our sins.
But this doesn’t mean that we throw up our hands in despair because righteousness is an impossibility. Instead we set it as the mark that we aim for and recognize that we won’t always hit that mark.
God has given us permanent righteousness in His eyes at the moment of our salvation. There is nothing we can do to earn this and nothing can repay the debt we owe. But we still continue to sin in spite of all of this.
This is why we need to daily ask forgiveness for our sins as well. The sins that we commit when we fail to be righteous do not affect our salvation. But they do affect our relationship with God. Because we want a right relationship with God, we aim for righteousness. When we don’t achieve it, we ask for forgiveness.