In programmatic advertising, a Supply-Side Platform (SSP) is a software used by digital publishers to sell ad inventory programmatically to advertisers. It allows publishers to easily manage their inventory and maximize their earnings by connecting them with multiple demand-side platforms (DSPs), ad exchanges, and ad networks at once. By doing so, SSPs let publishers sell impressions to a greater pool of potential buyers and allows suppliers to set the bidding range to maximize their revenue. SSPs provide similar functionality and technology as DSPs, but are used by publishers to help optimize and get the best offer for their inventory (aka maximize yield).
In programmatic ad buying, when consumers click on a publisher’s website, the publisher puts the ad impression up for auction through header bidding and one or more SSPs. Then, the DSP bids on behalf of the advertiser for that impression based on campaign’s strategies, budget, creative sizes, and other factors. The publisher automatically assigns impressions to the winning bidder—the advertiser/DSP offering the highest CPM (cost per mille, or the cost per one thousand advertising impressions).