This can mean a number of things.
Say you buy an ad in a local newspaper, the sponser's name is in your ad, the paper has a circulation of 50,000 that day= 50,ooo ad impressions.
Put your sponser's name on your house tickets, sell 20,000 tickets= same thing.20,000 impressions.
Print posters for your house add the sponsers name and/or logo, how many posters did you distribute?
Paint up a demolition derby car with ads , how many people were in the stands that night watching the race? How many people bought a newspaper that had a photo of the car with the ad?
Alot of the counting is educated guesswork.
You go nuts , take your scope rifle into the high tower of your haunt, it's a Police/Swat stand off sort of thing for several hours, the whole time CNN is watching and listening to you as you keep screaming "Lttle Fratino's Pizza drove me crazy, it tastes so good I can't get enough!"
Of course they will never again be your sponser.......much-o impressions though.