A duty of care is a reasonable duty of care to protect the plaintiff from certain types of harm, to not expose a plaintiff to certain types of harm. Examples of a duty of care would be the duty to drive the speed limit, if that was the safe speed to drive. However, if traffic is 45 mph on the freeway and the speed limit is 65 and you still drive at 65 mph when it is 45 mph, well you are not abiding by reasonable duty of care. So just because there may be a law commanding you to do something or giving you a limitation, you have to exercise your own judgment in order to avoid causing harm. That is basically what a duty of care is, a duty to abide by a reasonable standard of conduct that will avoid subjecting people to harm.