Frequency Capping allows you to choose how many times you want to show your ad to a unique visitor during a 24h period. Remember that a low cap may lead to less impressions on your campaign.
Advertisers can put frequency capping per banner, not per campaign. When creating/editing your campaign, under the « Ads » tab you can change the frequency capping from 1 to 5 or have no capping. The default setting is “no capping”.