Kant wrote that, when making a choice, there is only one option that is inherently good. He called this option the "good will." He stated that in order for the choice to be good, a person has to act out of a sense of moral duty, rather than out of a selfish inclination. If you make a choice based on an inclination--such as the opportunity to gain something or to feel good--then the act is meaningless.
The categorical imperative is linked to the notion of moral or ethical duty. It's a test to check if you're acting according to moral duty. Kant said, before making a decision, you should first generalize the principal on which you're about to act. Then ask yourself if the principal still makes sense or if it contradicts itself. If it doesn't make sense, then the action is morally wrong. If it does make sense, ask yourself if you'd want to live in a world where everyone followed that principal. If you wouldn't, then you shouldn't follow it.
In Kant's philosophy, he equated God with the idea of moral duty. He wrote that you couldn't separate the notion of God from moral duty or the ideal of a supreme good. He believed that, although one should try to always act according to an ethical duty, there was no possibility of achieving moral perfection in this life, as morality sometimes conflicts with our own safety or happiness. But since he thought that humans have an immortal soul, there would be time to perfect our morality.
Critics have argued that Kant's theories on moral duty assume that people have a shared moral compass and that everyone will make the same decision about what is right or wrong, where others argue that some people are innately more virtuous than others. It has also been criticized for rejecting the notion of making a decision based on the potential good that could come from the action, asserting instead that you should act only based on duty.