The Drupal API is the collection of functions, classes, services, and interfaces that modules and themes use to interact with Drupal core. It is documented at api.drupal.org.
The API covers everything from the render system and entity operations to form handling, routing, theming, and caching. Every major subsystem has a public API that other code can rely on.
Drupal’s API follows semantic versioning for major releases. Within a major version, the API remains stable, meaning code written for Drupal 11.0 generally works on 11.5. Breaking changes are reserved for new major versions like 11 to 12. This stability is one of the reasons Drupal is trusted for long term projects. Learning your way around api.drupal.org is a key skill for any serious Drupal developer.