It refers to an advertisement being displayed online.

However to simplify the entire case scenario, single instance is generally taken into count. Many people tend to make their purchases based on the impressions made every single day. For a layman, the term “impression” can be explained as something that both seller and buyer agree upon mutually. Impressions are calculated by relying upon ad servers. These servers are programmed in a way that excludes activities like reloading and so on that both Ad Company and advertiser agreed not to count upon.