Ad calls are an important part of the online advertising process. They allow the ad server or ad exchange to receive the necessary information from the user's browser and use that information to serve the appropriate ad to the user. Ad calls typically contain information such as the user's location, the publisher's ID, the size of the ad slot, and the referring URL.

When an ad call is made, the ad server or ad exchange uses this information to determine which ad to serve to the user. This decision is typically made using algorithms and is based on factors such as the user's demographic information, their browsing history, and the type of device they are using.

Ad calls are an essential part of the online advertising ecosystem. They enable advertisers to target specific audience segments and to track the performance of their ads, and they allow publishers to monetize their ad inventory and maximize their revenue from ad sales.