API Design is the job of determining the method/endpoints that an API will have, the structure and parameters of the request messages they will receive, the status codes and content they can return.
The output of this design activity is the API Spec File. These files are a contract between API Consumer and API Developer. API Consumer develops his own application using information such as to which address he can send a request, what possible responses can be returned. The API Developer, on the other hand, performs the operations that fill behind the definitions in this definition file, in other words, develops the API.