WordPress Core Application Programming Interface, commonly abbreviated as WordPress Core APIs is a collection of several individual APIs and each API covers the functions involved in adding a particular set of functionality to WordPress. Together, they provide an interface for WordPress developers to write plugins and themes which can interact with the core. ↔️
Using these APIs, you can leave the heavy lifting to the WordPress Core and be assured that your code is backward-compatible and future-proof.
Now that you have understood what WordPress Core APIs are, you can have a look at the below APIs to understand how to extend the functionality offered by WordPress through plugins.