An Application Programming Interface is a defined way for software to talk to other software: one system makes a structured request, the other returns a structured response. AI providers expose their models through APIs so developers can build applications on top of them.
APIs are how AI capabilities move from a lab's demo into real products. When a company 'integrates AI', it almost always means calling a model provider's API.
AI Panic's market data arrives through a provider API: the server requests end-of-day prices once per trading day and stores the response for all readers.