A courtesy is a polite remark or respectful act. Complain about a bad meal, and you might get kicked out. But the common courtesy is usually an apology from the manager and, if you're lucky, a free dinner.
Courtesy is all about using your good manners, which is why it shares roots with the word courteous. Holding the door open for someone, writing a thank-you note for a gift, and letting the pregnant lady have the last seat on the bus are all courtesies that would make your parents proud. And if something is kindly presented to you free of charge, the gift-giver may say it's "courtesy of" someone special.