A categorical imperative would be one which represented an action as objectively necessary in itself, without reference to any other purpose.
Kant is one of the giants of Western philosophy. Someone once said that philosophers who came before Kant had the enormous benefit of never having to study Kant. It is true that Kant’s philosophy can be heavy going to read and understand, but his theory of the categorical imperative is one deserving study. A categorical imperative would be something like ‘It is never right to lie.’ The test of the categorical imperative is whether it should be universally used. So, if lying became universal then trust would be abolished, therefore lying is wrong in all cases. Whether you agree with categorical imperatives or not, and situational ethics is the flavor of the modern age, we must all consider why things may or may not be good.