An application programming interface (API) is a software interface that provides a service to another piece of software. The instructions for how to use the API is called an API specification. When a programmer uses an API, they call the API methods. The API defines the method calls and how to use them.

Java Foundation Classes (JFC) is an API that provides services to this game engine and which you may want to use when creating your game. JFC bundles graphical user interface (GUI) components and services including Java AWT, Java Swing, and Java 2D.

Read more in Chapters 17 and 18 of Absolute Java by Walter Savitch.