API, for application programming interface, is one of those acronyms that is used everywhere from command-line tools to enterprise Java code to Ruby on Rails web apps. 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.
Diving a little deeper, an API is a specification of possible interactions with a software component. For example, if a car was a software component, its API would include information about the ability to accelerate, brake, and turn on the radio.