What are ad impressions?
An ad impression is the number of times your ad is shown on a search result page or other site on the Google Network. Each time your ad appears on Google or the Google Network, it's counted as one impression. The functionality of ad impressions is basically to know how many potential eyeballs you can have on your online ad.
Here are a few other definitions :
Viewable impressions: For display ads and videos, your ad is counted as viewable when at least 50% of its area is visible on the screen for at least 1 second. For large display ads formats of 242,500 pixels or more, the ad is counted as viewable when at least 30% of it’s area is visible for at least 1 second.
Served impressions: An impression is counted when an ad is sent (or "served") to a publisher by the ad server. The ad creative may or may not be downloaded to the user’s device, rendered, or viewed by the user.
The impression count can be used in your marketing campaign to know how many eyeballs have the potential of seeing your PPC ad. However, always take into consideration non-human traffic and interactions caused by bots.