An API (Application Programming Interface) is a request-response method that allows two systems to communicate by sending and receiving data in a format that both systems can understand. 

A simple example of an API in action is what occurs when you enter a web address in your browser or click a link. The browser takes the entered or clicked URL and sends a Request to the server asking it for the information of the page you would like to view. The server sends back a Response containing the web page in a format that your browser understands and can render for you on the screen.