A impression is simply the point in which an ad is viewed of displayed on a web page. The term impression can also be referred to a view or an ad view. The total number of impressions of a specific ad is determined by the amount of times the web page is located and loaded. If the ad is generated randomly, then this impression is determined by the number of times the ad appears from the random generator. An impression is helpful due to the fact that it let's you know the number of people your current ad is reaching, as well as the number of times the web page with the ad is being loaded. Typically, a campaign with a good impression rate, in fact has a high conversion rate as well.
E.g. “I’m getting a lot of impressions on my baby photos ad, but not many people are clicking - I may need to rewrite it.”