The source code of story of an API. This is the simplest explanation of what an API is. The blog post (link below) uses an adapted story of Life of PI and a java program to explain what an API is. The story also explain the need for Javadoc and API documentation.
For context and explanation, see the blog post: https://thecontentgym.wordpress.com/2019/04/08/life-of-api/
For an even simpler explanation of what an API is, see the video: https://www.youtube.com/watch?v=RMH9eO6abyI