Mill states that a pleasure is better than another pleasure when at least almost all people who have experienced both prefer one pleasure to the other, regardless of what they are supposed to prefer.
The utilitarian argument that the morally correct action is the one that promotes the most pleasure to the most people sounds great. In theory this would lead to the most people experiencing the most pleasure throughout their life. One thing most people can agree on is that pleasure is good. The problem with this argument is that when you apply it to real life situations, it begins to weaken. An example to strengthen the utilitarian position could be one of a doctor. In this example there is a terrible epidemic ravaging the world. The disease will kill billions if you are unable to find a cure. You can discover a cure but to do so, you must kill one of your own patients in the process. The patient who will die is currently in a medically induced coma and is unable to consent or communicate with you before you make your decision. Here the question is would you kill one person to save one billion. Easy