Frequency capping describes the process of restricting the number of times a visitor to the publisher's website will see a specific advertisement in a set time frame. This helps to prevent what is known as banner or ad blindness - many people will simply ignore ads if they are overexposed to them which will can damage the efficiency of you campaign.
Frequency capping on AdEx Network will be set by default to all campaigns which leads to proper ad rotation: every few minutes, a different ad is displayed. But you will have the option to disable it when creating a new campaign or a new audience in the advanced tab if you so wish.
Most campaigns on AdEx Network will have frequency capping set to 15 minutes. Ad slots also have a so called 'stickiness' of 2 minutes, meaning that they can only run one auction every 2 minutes and must display the result of the auction for this time. So no matter how many times an ad slot is refreshed, it will display roughly 8 campaigns in 15 minutes (if there are that many campaigns matching the ad slot).