A Supply-Side-Platform (SSP) is a technology platform that allows publishers to sell their advertising inventory in an automated way. SSPs are most often used by online publishers to help them sell display-, video- and mobile ads.
A Supply-Side-Platform basically is the publisher equivalent of a Demand-Side-Platform (DSP). While marketers use DSPs to buy ad impressions from exchanges as cheap as possible, publishers use SSPs to maximize the prices they can achieve for their inventories.
SSPs allow publishers to offer their inventories across multiple ad exchanges, DSPs, and networks at once. This way the publishers inventory is made available to a wide range of potential buyers. As this leads to an increased competition, publisher can sell their impressions for the highest possible rates.
When an SSP passes on impressions into ad exchanges, DSPs start to analyze and purchase them on behalf of the marketer depending on their desired attributes.
The basic idea of this whole system is that by making inventories available to as many potential buyers as possible so publishers can maximize the revenue they receive for their inventories.