The categorical imperative is an idea that Immanuel Kant had about ethics. An 'imperative' is something that one must do. The categorical imperative says that we should act in ways that we can wish that every person would act. The way he said that we should decide if we should do something is to us a maxim. A maxim is another way of saying what we want to do and why we want to do it in one sentence. Then he says to see if it would make sense if everyone had to do it. If the maxim is "I will lie to get out of trouble", then that would not make sense because if everyone lied, we would not ever know if what someone said was right. then it would not matter if you said anything because it would have no meaning. The most important part about Kant's idea is that we should not worry about what happens after we make a choice, because that does not make it right or wrong. Kant thought that another way of saying the same idea is to say that we should treat other persons as persons and not as tools that can help us in some way. It comes into Absolutism because it is based on duty and a universal law.
Kant thought that this idea is the basis to treat other people in the right way.