Utilitarianism depends on something known as consequentialism. In other words, a utilitarian chooses the action that leads to the "greatest good for the greatest number." Rather than, as would a deontologist, choosing an act based on the intrinsic merits of the act itself and a pure sense of moral duty, the utilitarian must estimate the outcomes of an act.
Take, for example, charity. A Christian might give money to a beggar under the moral precept that Jesus has commanded that one help the poor and needy. A utilitarian would ask whether that would be a good deed on the basis of whether it would lead to a greater good for a greater number. If one sees a poor person panhandling on the street, one does not have a way to know whether the person is trying to buy food for her children or will spend the money on alcohol or drugs. Even more, one does not know whether giving the cash to a beggar would be a more or less effective way to end hunger and homelessness than donating to a homeless shelter or food bank.
If one refrains from contributing to charity until one can figure out the utilitarian consequences, people may suffer from neglect, and yet one can never perfectly foretell the consequences of one's actions. Refraining from charity, whether helping the homeless or dealing with species extinction or animal cruelty, can mean increased suffering while one tries o gather data. In fact, this issue of perfecting information can become an excuse not to do one's moral duty to help the unfortunate who may need help before one has perfect knowledge.