Impressions represent the number of times an ad or a page is displayed to users through third-party channels, such as Ads, Google Network, Social media ads, or in SERP (Search Engine Result Pages).
Impressions represent the number of times that an advertisement has been shown through Google AdWords, Google Network, and other forms of PPC advertising or the number of times a link is displayed on a SERP.
An impression is counted:
Reach, as a metric, represents the total number of people who see your content or your ad. While Impressions are the number of times the content is displayed, no matter if it was shown to the same person multiple times or if it was clicked or not.