The term API is an acronym and stands for “Application Programming Interface”. Think of an API as a menu in a restaurant. The menu provides a list of dishes that you can order, along with a description of each dish. When you indicate which menu items you want, the restaurant’s kitchen does the work and provides you with a number of finished dishes. You don’t know exactly how the restaurant prepares that food, and you don’t really need to.
An API lists a number of operations that developers can use, along with a description of what they do. The developer doesn’t necessarily need to know how, for example, an operating system built a “Save As” dialog. They just need to know that it is available for use in their app.
This is not a perfect metaphor, as developers have to provide their own data to the API to get results. Perhaps it is more like a restaurant where you can provide some of your own ingredients that the kitchen will work with.