API calls are messages sent from a client to a service, similar to mailing a letter where the endpoint acts as a mailbox or destination address. To build an API call, you need to supply an address, action, header, and optional data, which is then processed by the service that constantly listens for requests. The service retrieves the request, unpacks parameters and data, and returns a response to the client. APIs can be used with various programming languages, including those built into macOS and Linux, and tools like Postman provide a user-friendly interface for testing public APIs. Understanding how to make API calls is essential for building digital experiences that power tech we use every day.