What it basically boils down to for Kant is "duty" -- you ought to act the way you'd expect everyone else to act in the same situation.

The whole idea is that desired ethical behavior is that which could be applied universally to everyone, no exceptions.

You don't steal, because you wouldn't like it if everyone else did the same. You don't lie, because you wouldn't like it if everyone else lied as a matter of course.

In a way, it's just a colorful play on the "Golden Rule" (do unto others...) which is pretty much a universal maxim across almost all religious boundaries, clear throughout history, including secular humanists and existentialists today.

The one major weakness in his ideology is that some people are masochists, and don't care if others get hurt because by their actions because they don't mind enduring some pain themselves. So they could justify harm by claiming they don't care if everyone else did the same, which leads itself to moral subjectivism, everyone doing whatever they feel like with no way to objectively judge them.

In the end, a comprehensive modern ethical theory would have to blend the best of all worlds, some of the best ideas of a multitude of philosophical thinkers throughout history. Some have dubbed such an idea "soft universalism" (one universal code of laws that everyone should abide by, but with enough flexibility to adapt to changing times.)

It's been a fascinating class so far, that's for sure.