Let's say, for example, that you have a page that has five graphics on it. That page is equal to six hits, the page and then the five graphics. This is wrong, as it creates an inflated report of site traffic. If I could report hits alone, I could create graphics-filled pages and report numbers into the tens of millions. It may be that one person saw the ad banner three times.
Impressions are simply the number of times the banner was seen.
Usually this number is reported right alongside Impressions. Something like, HTML Goodies creates 3 million impressions a month from 1.1 million individual visitors. When I first started Goodies, I made a point of making my pages one hit each, three at the most. The reason was that I sold by impressions, but I payed by hits. I wanted the two numbers as equal as possible.