While reach is the measure of the audience for your ads, impressions measure how often your ads have been displayed. This also means that impressions can be, and often are, much higher than your reach, since an ad can be displayed multiple times to consumers. At that point, consider audience fatigue: Seeing the same ad over and over again is not the best customer experience.
To help prevent this, consider frequency capping to limit the number of times each of your ads is shown to customers. Frequency is the number of times your ad is shown, or its number of impressions, per individual. Since you don’t want to inundate your customers with repetitive ads within a short time frame, frequency capping allows you to create a cutoff for your ad impressions.