The shoulder block or shoulder roll is a defensive boxing technique. As its name implies, boxers use it to deflect punches with their shoulder. Its primary purpose is to make a very efficient defense as possible while not completing committing to it. This would give the boxer huge area for other elements while having a good defense.
Shoulder roll is done by putting one of the boxer's forearm (preferrably the dominant one) around the side part of their pelvis. The shoulder would them be rolled up to cover the jaw. This kind of efficient defense protects two of the most targetted areas of the body in boxing: liver and jaw.
Generally shoulder-roll users use the shoulder to block punches and because the rest of the body are free, unlike turtle style for instance, the boxer could easily do other things immediately like counter punching or positioning themselves for a better attack. Floyd Mayweather, for example, uses the shoulder to parry opponents' punches to make them glancing blow then quickly throw an effective counterpunch. David Eagle also showed this strategy in his fight with Mamoru Takamura.