When a user opens a page or app, a bid request describing that impression (GEO, device, context and more) is sent to buyers. Each DSP decides in real time whether to bid and how much, based on the advertiser's targeting and bids. The highest relevant bid wins, and that ad is served – all before the page finishes loading.
This happens billions of times a day across the open web and apps. For advertisers, it means you can buy exactly the impressions you want, one at a time, instead of buying inventory in bulk.