Api is a program or a collection of datas which is purposely developed to be used by other programs for example if u want to develop an app which shows wether condition u have to know the weather condition in every county but as you might have guessed u can't put a machine in every country so the solution is using api . The will gives u information about wether condition in every country . Generally api is a service that u may include to ur application to add more functionality to your application.