A supply-side platform is an advertising technology where publishers sell and manage their ad spaces to monetize their mobile apps. It is one of the parties involved in Real-Time Bidding. 

An SSP provides a dashboard to let publishers connect their apps, define their ad placements, floor prices, requirements for the ad formats, and other controls. Via SSPs, publishers sell their ad spaces to ad exchanges, DSPs, and ad networks. Publishers can also automate their selling process to save time and resources. 

A supply side platform is an equivalent of the demand-side platform (DSP).