What is an API? In English, please.
Petr Gazarov

As interesting as this article is, it’s overkill. In plain English, an API is an agreed upon way for code to interact with a library of public methods. There is usually a documentation available that includes a list of the methods available, a description of what parameters must be passed to get the desired result, and a description of how to interpret the returned values.

For example, if I create a library of code to access a movie database, I can publish documentation for the interface to that library so other people can access the data. That is an API.