Advertising: Using weights
The search engine selects an advertisement at random to display to the user. Weights allow a certain ad to show up more often.
For example, if 3 ads have the default weight of 100, then they will be shown about the same number of times (assuming for the moment that no keywords are defined), that is, each ad will show up 33% of the time. If the first ad has its weight increased to 200, then it will now show up half the time. The formula for determining frequency is the weight of a given ad divided by the weight of all ads, or 200 / (200 + 100 + 100) = 200 / 400 = 50% in this example.
Weights are typically used when a single advertiser provides multiple banners. Assume Bob and Jane have both been promised 50% of the ad impressions, but Bob provides ten banners and Jane provides just one. Jane will have a weight of 100, and each of Bob's banners will have a weight of 10. Each customer will receive half the impressions.
"Advertising: Using weights" https://www.xav.com/scripts/search/help/1042.html