Utilitarianism is a normative ethical theory. Normative ethical theories tell you what to do (or, in the case of virtue ethics, tell you what kind of person to be). In the specific case of utilitarianism, it holds that the right thing to do (i.e. what you ought to do) is maximize world utility. In the current world, there are many people who could sacrifice a lot to generate even more world utility. Utilitarianism holds that they should do so, therefore it is demanding.