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. 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.