An impression occurs when an advertisement or any other form of digital media renders on a user’s screen.
It's The number of times your ads were on screen. This does not always mean that a user actually viewed the ad or listing.
An impression is counted as the number of times an instance of an ad is on screen for the first time. (Example: If an ad is on screen and someone scrolls down, and then scrolls back up to the same ad, that counts as 1 impression. If an ad is on screen for someone 2 different times in a day, that counts as 2 impressions.) Impressions are a social media metric that measures how many times your post has been shown in users’ feeds. Unlike with reach, you may count multiple impressions for a single user if they have looked at your post more than once. Each social network counts impressions differently—on Facebook and Instagram a post.