The App Store Connect API is a REST API (an interface that programs call over HTTPS with standard web requests) from Apple. It automates the actions a developer would otherwise do by hand in App Store Connect, Xcode and the Certificates, Identifiers & Profiles site.
Each call is authorized with a JSON Web Token (JWT), a signed token that the program creates from a key issued in the organization's App Store Connect account. Responses are JSON data with links to related resources. The API covers in-app purchases and subscriptions, TestFlight beta testing, app metadata and releases, provisioning (bundle IDs, signing certificates, devices), users and roles, Xcode Cloud builds and customer reviews. Its reporting part downloads sales and financial reports, analytics reports and power and performance metrics, so outside systems can read an app's sales without access to the console. Apple publishes an OpenAPI specification of the API.
Access comes with the paid Apple Developer Program membership; the API itself has no separate price.