Display rules are a social group or culture's informal norms about how to appropriately express emotions. The norms of a group not only identify when and where it is appropriate to express emotions but also the extent at which these emotions are expressed.
In some instances social norms dictate when emotional displays should be suppressed. In most cultures it is appropriate to cry and show negative emotions at a funeral while at a wedding negative and sad emotions should be masked or suppressed. Display rules are often used as a way to protect the feelings of oneself or other people.
An example would be masking your true feelings about your friend's terrible cooking or being friendly to your opponent after losing an important competition.