Impressions are sometimes called ad views. It is a term that refers to the point in which an ad is viewed or displayed. Impressions are an estimate of the number of people the advertisement is appealing to and may be calculated in different ways such as the number of times the web page where the ad is shown and the location of the ad on the page. Depending on where and how an ad appears on a page, the advertisement might receive one or two impressions: a vertically listed advertisement will likely get two impressions as the visitor has to scroll down to see the particular advertisement.
A good indicator of working marketing campaigns is tracking the number of impressions. A large number of impressions and a good conversion rate means that a campaign is working effectively. A campaign with a low number of impressions and conversion rate can be an indicator that strategies need to be evaluated and/or changed. It is important to keep in mind that entire marketing campaigns should not be changed specifically by looking at impressions as impression data can be skewed by multiple page views by one visitor.
No matter how varied they are, impressions are important to search marketing. However, in the end, advertisers will use other forms of reporting to decide whether their entire campaign is working or not.