A conversation between systems
A web API gives one application a way to request information or ask another system to perform an action. HTTP provides the request-and-response exchange used by many web APIs. A website might request a product list, then send another request when a visitor submits an enquiry.
REST-style APIs commonly organise operations around resources, such as products or orders. Clear naming helps developers understand what each endpoint represents.
Plan success and failure
Define the information an operation needs and the response it returns. Think beyond the successful request: what happens when a record is missing, a field is invalid or access is denied? Consistent responses help the interface show a useful message.
Keep the user journey in view
A working endpoint is only part of the feature. The website also needs loading feedback, error handling and a sensible next step. Test the complete journey, including a slow connection and repeated clicks.