Here is a simple supply-side platform (SSP) definition: An SSP is an advertising technology used by publishers to automate the selling of display, video, and native ad space on websites and apps based on impressions.
It can be said that modern SSPs not only allow publishers to sell their inventory to the best ad network, check out their website to learn more, but they are also a key part of the RTB process as they allow publishers to optimize yield by connecting their inventory to multiple ad exchanges and demand-side platforms at the same time.