Sell-side ad tech is the software that publishers use to sell the empty ad space on their websites, apps and streaming services. A publisher is any business that owns pages or screens that people look at, such as a news site, a game or a streaming app. The sell side is also called the supply side, because it supplies the ad space that advertisers buy.
Imagine a free recipe website with lakhs of readers a day. Every time a reader opens a recipe, the page has three empty ad slots. One showing of one ad to one reader is called an impression, and the site earns a small amount for each impression. It has less than a second to sell each one, so software does the selling.
A supply-side platform (SSP) is the publisher's selling software. It offers each impression to many buyers at once. It also sets a floor price, the lowest price the publisher will accept, so that the space is not sold too cheaply. Many websites also add header bidding, a small piece of code at the top of the page that asks several SSPs for bids at the same time. The auction itself runs on an ad exchange, where the sellers' software meets the buyers' software. The buyers bid through demand-side platforms (DSPs).
Some sellers choose the ad by the page instead of the person. A contextual ad network reads the words and images on a page and places a matching ad, such as an ad for cooking pans beside a recipe. This method is explained in contextual advertising. Native advertising makes ads that look like the content around them, such as a sponsored article in a news feed.
App makers have their own tools. In-app advertising monetisation software offers each ad slot inside an app to the ad networks that pay the most for it.
Not every sale is an auction. Large deals, such as a season of ad spots during cricket matches on a TV channel, are still agreed between people. They are recorded, scheduled and billed in an ad order management system.
The sell side is under pressure. Search engines now answer many questions with an AI summary, so fewer readers click through to publishers' pages. Some publishers now also earn by licensing their articles to the companies that train AI models. And Google, Meta and Amazon, the walled gardens, keep a large part of all ad budgets inside their own apps.