Let’s consider a few examples.

Kant begins with an example of a shop keeper. He wants to bring out the intuition and make plausible the idea that what confers moral worth on an action if that it be done because it’s right. He says suppose there’s a shop keeper and an inexperienced customer comes in. The shop keeper knows that he could give the customer the wrong change, could shortchange the customer, and get away with it. At least that customer wouldn’t know. But the shop keeper nonetheless says well if I shortchange this customer, word may get out, my reputation would be damaged and I would lose business, so I won’t shortchange this customer. The shop keeper does nothing wrong. He gives a correct change. But does this action have moral worth? Kant says no. It doesn’t have moral worth because the shop keeper only did the right thing for the wrong reason out of self-interest.