API stands for Application Protocol Interface (or Application Programming Interface), which is more simply a set of clearly defined methods of communication between various software components that are used to build apps.
Think of an API as a mailman who sends your app’s request to a provider (some other software), and then delivers the response back to your app.
A simple example would be an API request to update your appointment calendar when you confirm your attendance from an emailed meeting invitation. The API allows your calendar software to work and synchronize your appointments from within your email software.
To make a long story short: an API essentially bridges the divide between two separate applications so that they can talk to, and work with, one another.
But why is this important?
Well, no one wants to comb through your app’s code – no matter how neatly it’s written –just to find a specific function. APIs open up specific functions within your app that other software can use to do its job. The API is what allows apps to share data and take specific actions.