Frequency capping gives advertisers the ability to specify a limit to the number of:
impressions for all viewers
impressions for a unique viewer
clicks for a unique viewer
clicks for all viewers
Explanation:
Repeating explication from previous question. Frequency is the average number of times a unique user sees your ad in a position of “1” 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 on the Display Network When you turn on frequency capping for a campaign, you set a limit for the number of impressions you will allow an individual user to have per day, per week, or per month. You also choose whether the limit applies to each ad, ad group, or campaign.
Frequency is the average number of times a unique user sees your ad in a position of “1” 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 on the Display Network.
Read more here: https://support.google.com/adwords/answer/117579?hl=en