The subscapularis is a large triangular muscle found on the inside of the shoulder blade. It is the last of the rotator cuff muscles, and is the only rotator cuff muscle that attaches to the lesser tubercle of the humerus (a slightly smaller knob below the head of the humerus). And now that we have identified all of the rotator cuff muscles, we can introduce a handy acronym to remember them: SITS. Supraspinatus. Infraspinatus. Teres minor. Subscapularis.
The subscapularis is the only rotator cuff muscle that performs internal rotation of the shoulder. But it also helps during abduction and adduction; and when the arm is raised, it helps to pull the humerus down and forward, keeping the shoulder socket stable.
In order to target the subscapularis, we should generally perform internal rotation exercises, much like mentioned before. You will notice that most movements of the shoulder will work one or more of the rotator cuff muscles, so by incorporating a few of these each week, you can effectively train all four. Of particular interest for the subscapularis will be keeping the shoulder socket stable (avoiding shrugging) during any overhead movements, whether with a barbell or dumbbells (or even kettlebells, why not?).