A good pitcher can pitch a fastball at speeds upward of 100 miles per hour (161 kilometers per hour) and when something's moving at 100 mph, you just know there's a chance someone's going to get hurt. More often than not, the person that gets hurt is the pitcher.
The rotator cuff is the set of four muscles that allow the human shoulder to, well, rotate. The shoulder can rotate into more positions than any other joint in the human body and the rotator cuff is one of the reasons why. Like other sets of muscles in the body, the rotator cuff muscles are held together by bands of connective tissue called tendons. Repetitive stress on these tendons, like using the shoulder to throw fastballs at speeds that would be illegal on most highways, can cause them to become inflamed and eventually to tear. This usually occurs over a long period of time and can be prevented by performing warming up exercises for the shoulder muscles before the serious pitching begins.
Once a tendon tears, the pitcher will experience pain and an inability to rotate the shoulder properly, both of which are bad news, especially if you make your living throwing baseballs. Fortunately, there are same-day surgical techniques that can repair the damage and medicines that can help the pain. Mostly, though, the injury just needs time to heal.