Kant's ethical theory was evidently a product of its time and Kant's reaction to that time, in which great philosophical unheaval was taking place.
Kant began like Descartes as a rationalist, believing we know truth through the mind (opposed to human knowledge comes from experience like Locke and Hume). Kant eventuall moved to a midway position between Rationalism and Empiricism.
A Priori - knowledge comes from reason without the need for experience
A Posteriori - knowledge only comes from experience
Analytic knoweldge - the predicates may say something that is necessarily true about the subjects. eg all widows are women. The statement required no further facts from an exterior source. It is necessaarily true - true by its own authority.
Synthetic knowledge - may require empirical tests, such as observations, measurements of experiments. eg. Jack is a butler. The statement isn't necessarily true because we need exterior information to support the claim that he is a butler. Synthetic statements can be true or false. The statement is also posteriori as it is knowable only from experience.
His main areas of study was reasoning, causality, a priori knowledge and objectivity.