Impressions are the amount of times the ad is viewed from your site. Generally you don't get paid for impressions if so it's usually like 3-4 cents per 1,000 impressions on that Ad
that is correct. that's the concept of CPM. Most of the company like MobGold, Admobs pays per click though.
if u use some ads such as advpoints then you will get paid from that impression around $1-$2 per thousand impression.