What is an IMPRESSION?
An impression is counted each time an ad loads on a site. If you refresh the page, even if the same ad loads, a new impression is counted.
Why are IMPRESSIONS important?
For advertisers, impressions are important because they tell you how many eyes have seen a particular ad. You can calculate the reach of the ad based on how many times it was seen.
For publishers, impressions are important because in the CPM (Cost Per Mille) advertising model, you are paid for every 1000 impressions of an ad served on your site.
What is the difference between SERVED IMPRESSIONS and REQUESTED IMPRESSIONS?
Served impressions refer to the number of times an ad is successfully served on a site. Requested impressions refer to the number of times a page loads but an ad is not served. In online advertising, ad space inventory lasts only a fraction of a second. If an ad is not in stock and ready to fill the ad space, the ad space inventory goes unsold. When network fill rates are low (meaning we have more publisher ad space than advertisers), requested impressions will be higher.