An impression is the number of times one of your visitors had viewed a page where a video or audio was included.
It does not mean the media was actually played by the visitor. As soon as the visitor starts the video or audio, a “play” event is tracked. The ratio of how many users play your content VS how many users have loaded a page with the content included in the page, is represented as the “play rate”. For example if 100 visitors have been on a page that included a particular video, and 30 of it have actually started it, the play rate is “30%”.