The setting prevents a visitor from seeing a specific placement too often. Seen too often already → no more display, default code is returned Not yet seen too often → capping is incremented and the ad can continue to run
Campaign capping is an ad placement setting on the General tab
Campaign capping defines how many views a user is allowed to see within a given time period x. This applies to all campaigns combined. IMPORTANT is that both fields, Views per... and ...time period, are selected.
Example:
Entry 3 per hour → the user will see a maximum of exactly 3 views per hour on this ad placement (campaigns are delivered according to the usual criteria for views).