A definition with teeth.
Professors will have you study this forever. Here's what you need to know. It means what it sounds like: an imperative (commandment) that you either follow, or you'll be reclassified as immoral.
Kant had his, and many people think his was the only one, but it's not true. A categorical imperative is a definition-based morality. Even the Utilitarians use them.
What will happen if I do something immoral?
You'll have done something immoral.
Oh, it's a categorical imperative.