What is an API?
An API is a standardized way of interacting with a software application. APIs allow different software applications to interact with each other without having to understand the inner workings of the underlying functions.
APIs have become popular in modern software engineering as they allow organizations to be more effective by reusing code. Take the use of maps as an example: before the popularization of APIs, organizations needing map functionality would have to maintain map widgets internally. Often, these map widgets would perform poorly, as they were only a secondary concern of their business and engineering teams.
Now it's rare for a website or application that uses a map to maintain it internally. Many applications for the web and mobile phones are utilizing map APIs from either Google or alternatives such as OpenStreetMap. This allows each company to focus on its core competency by not having to create and maintain their own map widgets.