Ad Displays or Impressions are the number of times your ad was on-screen.
This is a very common metric used in online advertising and helps you gauge how often your content is being placed in front of your target audience.
Facebook goes into more detail about how these metrics are calculated:
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 one impression. If an ad is on screen for someone two different times in a day, that counts as two impressions.) This method of counting video impressions differs from industry standards for video ads. With the exception of ads on Audience Network, impressions are counted the same way for ads that contain either images or video. This means that a video is not required to start playing for the impression to be counted.
In a few cases when it can't be determined whether ads are on screen, such as on feature mobile phones, impressions are counted when ads are delivered to devices.
Impressions aren't counted if they come from invalid traffic we detect such as from non-human sources (such as a bot).