Frequency capping is the average number of times a unique user sees your ad over a given time period. Use frequency capping to help control the maximum number of times that you'd like each person to see your ad

To do this, users can define the times you want the ad to be shown per unique user in the Frequency Capping field and the time interval that the frequency capping to apply in the Time Base field. (For example, 5 times per day). 

Tips: For niched target audience groups, too tight frequency capping could greatly decrease the delivery. It is better to set loose frequency capping or remove it completely to get enough reach. There is no best way to set frequency capping since the performance varies on individual campaign basis. It is advised that users can try to run the campaign without the frequency capping in the beginning of the campaign period in order to find the trend and optimize the campaign later.