With auto-refreshing, every advertising space on a website can serve multiple ads within a single page view. The ads are refreshed after regular intervals and their refresh rates could be based on a range of things including time durations or user actions like scrolling, clicking on the screen or using site search, etc.
Some publishers set low refresh intervals in an attempt to cram as many ads as possible in a single page view. Because the ad display duration is really short, the user might not register the ad but an impression will be registered regardless. When auto-refresh is misused in this way on a CPM model, advertisers end up paying for impressions on ads that were either unviewable or not in view for long enough to solicit the desired action from the audience.
Source: TrafficGuard Glossary