A supply-side platform (SSP) is the software that a publisher uses to sell its ad space automatically. A publisher is the owner of a website, app or streaming service where ads appear. For each impression, meaning one showing of one ad to one person, the SSP offers the space to many buyers at once, takes the best price and collects the money. Publishers are called the supply side, because they supply the ad space.
When a reader opens a page, the SSP receives a message for each empty ad slot. It turns each one into a bid request, a short description of the slot: the website, the size of the slot, the device and the rough location. It sends the request to several ad exchanges, and directly to demand-side platforms (DSPs), the buyers' bidding software. It also sets a floor price, the lowest bid that the publisher will accept, so that the space is not sold too cheaply. Then it picks the highest bid above the floor.
Consider a Tamil news website with about 2 crore page views a month. A reader opens a story about the monsoon. The SSP offers the top banner slot with a floor of ₹40 CPM, meaning ₹40 per thousand impressions. Within about 200 milliseconds, three bids return: ₹62, ₹55 and ₹38 CPM. The ₹38 bid is below the floor, so it is ignored. The ₹62 bid, from a two-wheeler brand, wins. The site earns 6.2 paise for this one impression, minus the SSP's share, which is often 10 to 20 per cent. Over 2 crore page views, with several slots on each page, these fractions of a rupee pay for the site's reporters and servers. The prices are explained in impressions and CPM.
The SSP also protects the publisher. It blocks ads that the publisher does not want, such as ads for a rival news site. It works with ad verification tools to reject traffic from bots, programs that pretend to be readers. Bots are used to create fake impressions and collect ad money, which is called ad fraud. On many websites, the SSP receives its requests through header bidding, code on the page that asks several SSPs at once.
The SSP is the main selling tool of a digital news publisher. Large deals that people agree by negotiation are handled separately, in an ad order management system.
SSPs are now moving into streaming TV, where server-side ad insertion (SSAI) asks them to fill each ad break. This matters because publishers' websites are getting fewer visitors, as search engines answer more questions themselves with AI summaries. Some SSPs also help publishers earn from AI content licensing, by selling AI companies the right to use their articles.