An impression is each time a banner ad is displayed on the site.
Advertising is usually sold in one of the following terms:
Time (how long it will be in rotation on the site - this is the least reliable because it doesn't tell you how many times your ad will be seen and how many other ads are in rotation with yours).
Click-Throughs (the number of times a user has clicked on an ad - this is a standard way of paying a small website to display banner ads, each time someone clicks on the ad and visits the target site, the source site gets a very small credit).
Impressions (the number of times the ad will be displayed for a set amount of money, which is how large sites sell advertising - this means it doesn't matter if the site goes off-line for a week in the middle of your campaign, the ads will still go on, you aren't charged for downtime. And you get the same number of adviews no matter how many other advertisers are buying adspace on the server).