Consequentialism is the name given to one school of thought as to what makes an action moral or immoral. As the term implies, this school believes that the morality of an action is determined by its consequences, not by the intentions of the actor.
For example, let us say that I start a program at my school that is aimed at helping students from poor families improve their chances of going to college. Let us say that I do this in an attempt to help them. You might think this is a moral action because my intentions are good.
But a consequentialist would say they do not know if the action is good until they see what consequences it has. Let's say that my program works, but that these students become estranged from their families because their families did not want them to go to college.
To the consequentialist, my action's morality is judged by balancing these effects -- my students went on to college, but now their families hate them. We have to decide whether the good outweighs the bad to understand whether my action is moral or not.