An important part of good defensive strategy is being able to recognize the formation of the offense, and predicting what plays they will run based on that formation. If you have studied your film, and know your opponent's tendencies, this can give you an edge.
The first thing you want to determine is whether the offense is weighted to one side or the other. This is where you will often hear the terms "strong side" and "weak side." So how do you determine which side is strong and which is weak?
Every offensive formation will have 5 eligible receivers plus a quarterback (except in the wildcat offense). As the offense lines up, the safeties and linebackers will immediately survey the formation and adjust their alignment appropriately.
Determining the passing strength of the formation basically comes down to seeing which side of the formation has more eligible receivers. With a few exceptions, if you split the formation in half at the center, whichever side has the greater number of backs and receivers is the strong side in terms of passing. Here's an example. Let's say you had one receiver left, two receivers and a tight end right, and the running back was behind the quarterback, we'd say that the passing strength was to the right.
Run strength is similar to passing strength. You're trying to predict where they are most likely to run, based on their formation. So, the linebackers and defensive linemen will look for the tight end and the alignment of the running backs. They will also split the formation in half at the center, and determine which side has the strongest run threat. If there were only one running back and one tight end, the run strength would be to the tight end side. If there are two tight ends, the strength would be to the side that the back is lined up on. Your coach will also let you know which side should be called in the case of a balanced run formation.
If you want to maintain an edge defensively, you have to know the formations and tendencies of your opponent. Finding the run and pass strength is the first step.