What is an SSP (supply-side platform)?

A Supply-Side Platform (SSP) is a platform designed for publishers, the owners of ad inventory. SSPs enable publishers to monetize their ad placements in real time. Typically, SSPs operate through Real-Time Bidding (RTB), allowing advertisers to bid for ad impressions in real-time auctions. However, SSPs can also operate through direct sales models, selling ad inventory directly to advertisers.