An impression is the number of times an ad instance appears on screen for the first time. (For instance, if an ad is on screen and someone scrolls down, then back up to the same ad, that counts as one impression. If an ad appears on someone’s screen twice in a day, that counts as two impressions.) This method of counting video impressions differs from industry video ad standards. With the exception of ads on Audience Network, impressions for ads containing images or video are measured in the same way. This means that a video does not have to begin playing in order for the impression to be counted.
The number of impressions you receive can vary depending on your audience size, budget size, ad relevance, quality score, and a variety of other criteria. If you want to improve this metric, it might be as simple as increasing your budget or as complex as reworking all of your advertisements to be more relevant to the audience.