OpenRouter is like a market place for AI model providers. So other companies can offer access to AI models through their servers, and OpenRouter will expose them to you via a unified interface. When you send a request to a certain AI model, OpenRouter forwards that request to the currently cheapest provider that’s available.

So it helps you get easy access to thousands of models for a competitive price, without having to pay tens of different providers and figuring out which is cheapest and such. It’s a convenience thing, mostly.