API stands for application programming interface, a concept that applies everywhere from command-line tools to enterprise Java code to Ruby on Rails web apps. An API is a way to programmatically interact with a separate software component or resource.
Unless you write every single line of code from scratch, you’re going to be interacting with external software components, each with its own API. Even if you do write something entirely from scratch, a well-designed software application will have internal APIs to help organize code and make components more reusable. And there are numerous public APIs that allow you to tap into functionality developed elsewhere over the web.