On CPM campaigns it's the number of times a unique visitor sees your ad.
You are given the possibility to choose from 1 to unlimited impressions / unique visitor / 24h.
Note that the frequency capping is set by variation, not by campaign.
So if you have for example 2 banners and you set your freq. cap to 3/24 hours, the user will see 6 impressions of your campaign in a 24 hour span.
Be careful when setting a high Frequency capping since you will spend your daily budget very quickly and may even exceed it.
Remember that an advertising message is normally displayed more than once as we know that the effectiveness of the message is based on repetition. By testing different type of campaigns and by playing with the cappings you’ll be able to fine tune your campaign to find the right exposure bringing higher profits.