Impression Share Bidding—The Act of Marketing.

[Jasz]

Impressions share bidding is a way of bidding on advertising space that helps marketers determine which ads to show and where. In the past, marketers used a cost-per-click model in which they paid for every click on their ad. This meant that if someone clicked on an ad, they would pay for that click regardless of whether or not it led to a purchase. However, this method did not account for the fact that some ads are more effective than others at getting people to click through and make purchases.

Impression share bidding allows marketers to bid on advertising space based on impression share rather than cost per click. In other words, marketers can now bid on ads based on how often they will be displayed in relation to other ads instead of just how often people will click on them after they appear in front of them.

The advantage is that it allows marketers to target their audience more effectively by showing their ads where they are most likely to be seen by people who are likely to respond positively when seeing them, and paying less money overall because they aren't paying for more clicks than necessary.
For example, if there are 50 total advertisers (competitors) competing for a single impression, and your ad is displayed two times while all other ads are only displayed once, then you’ve achieved a 100% impression share.