The performance of your defense depends mainly on their historical stats. The Defensive Focus is a way that allows you to bend their performance towards rush or pass for certain circumstances.
The defensive stats are an average over many games and many game situations, so it is not unreasonable to expect them to play tougher against the run, or pass, against some teams or for certain plays. The Defensive Focus settings allows you to say when they should be playing tougher.
Keep in mind, that you can't make a weak defense into a strong defense, and if you play tougher against the run, you will give up some against the pass, and vice versa.