The subscapularis is one of the four muscles that make up the rotator cuff group. The rotator cuffs are group of muscles that, along with their tendons, pretty much act to stabilise the shoulder. Of the four muscles that make up the rotator cuff group, the subscapularis is the most powerful.
The muscle itself originates from the inside of the scapula (shoulder blade) and then inserts itself on a small bump (lesser tuberosity) on the front of the arm (humerus). You can see this by viewing the image above.
The job of this rotator cuff is internal rotation (to turn the arm inwards). Problems and tightness in this muscle restricts shoulder function and can play a role in shoulder pain and discomfort although out of the four rotator cuf