Impressions are the number of times an ad is consumed (heard or seen).  The Impressions number is calculated by multiplying the number of people listening to the stream at the time of play by the number of spots played. It's the number of people streaming at the moment the ad is played.

For example, if there are 100 people listening to a radio station when a single spot plays, that spot play would generate "100 impressions", but only account for a single spot play.