Understanding the difference between Plays and Impressions
A Play corresponds to a reading of your video on the screen, independently of the number of people actually present or not in front of the screen.
Generally speaking, your video lasts 10sec and is played within a 60sec loop, at a rate of one play per loop.
An Impression corresponds to an opportunity to see your ad, i.e. to a person who has potentially been exposed to your video.