Courtesy bias is the tendency that some individuals have of not fully stating their unhappiness with a service or product because of a desire not to offend the person or organization that they are responding to. It is a tendency for a person to respond in a socially acceptable, polite, or positive way regardless of what they actually feel. Whether or not this strategy works depends on the nature of the problem and the attitude and presentation of the parties involved.
For instance, you go to a restaurant and have a horrible experience - your food tasted awful, your service was bad, etc. The courtesy bias would occur if, when asked by the manager how your experience was, you didn't want to cause the kitchen/server any trouble with their boss and responded that everything was wonderful. In order to be positive and polite your true feelings about the experience are not expressed.