The waterfall model is a software development method that involves a linear, sequential approach to the development process. It is called a "waterfall" model because each phase of the process flows logically and sequentially into the next, like a waterfall.

In the context of display advertising, waterfalling or the waterfall model is often used to describe the process of ad serving and ad buying. In this context, the model refers to the sequence of events that occurs when an ad is served to a user.

In a typical waterfall model for ad serving, the process starts with the publisher's ad server, which is responsible for managing the ad inventory and serving the ads to the user. The ad server then sends a request to the demand-side platform (DSP) of the advertiser, which is responsible for buying and managing ad inventory on behalf of the advertiser.

If the DSP is interested in the ad inventory, it will send a bid request back to the ad server. The ad server then conducts an auction to determine the winning bid and serves the ad to the user. The winning bid is then passed back to the DSP, which pays the publisher for the ad inventory.

The waterfall model for ad serving is a linear process, with each step flowing logically and sequentially into the next, which is the opposite of the real time bidding model that takes place in parallel. It is often used to manage the process of ad serving and ad buying in a way that is efficient and transparent.