An ad slot is the specific area on a website where an ad is displayed. Ad slots are typically designated by the publisher of the website, who chooses where on the page the ads will appear. Ad slots are typically defined by the size of the ad that will be displayed in them, such as 300x250 pixels or 728x90 pixels.

To load an ad on a webpage, the ad slot contains an ad tag, which is a piece of code that communicates with the ad server. The ad server uses the information provided by the ad tag 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 slots are an essential part of the online advertising ecosystem. They provide advertisers with the ability to target specific audiences and to track the performance of their ads, and they allow publishers to monetize their ad inventory and maximize their revenue from ad sales.