Application Programming Interface(API) is a set of requirements that govern how one application can talk to another and allows other programs to interact with it without the need for a developer to share its entire code.
If you have a smartphone, you are well acquainted with what applications are, i.e., the tools, games, social networks and other software that we use everyday.
Programming is how engineers create all the software that make our lives so much easier.
An interface is a common boundary shared by two applications or programs that allow both to communicate with one another.
So, an API is essentially a way for programmers to communicate with a certain application. Customer needs to expose an API that tells a programmer how they will interact with the service.