The QMK API provides an asynchronous API that Web and GUI tools can use to compile arbitrary keymaps for any keyboard supported by QMK. The stock keymap template supports all QMK keycodes that do not require supporting C code. Keyboard maintainers can supply their own custom templates to enable more functionality.
If you are an app developer interested in using this API in your application you should head over to Using The API.
If you would like to enhance your keyboard's support in the QMK Compiler API head over to the Keyboard Support section.
If you are interested in working on the API itself you should start by setting up a Development Environment, then check out Hacking On The API.