What does impressions mean?

Impressions are the number of times an ad was heard or seen.  impressions are calculated by multiplying the number of people listening to the stream 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.