What is an API?

API stands for Application Programming Interface a defined set of rules that lets two pieces of software talk to each other.

Think of a restaurant: you do not walk into the kitchen and cook. You give your order to a waiter (the API), the kitchen (server) prepares it, and the waiter brings the result. You never need to see the kitchen.

When a weather app shows today's temperature, it sends a request to a weather service's API "give me the temperature for London" and the API returns the data. The app displays it. The app never needs access to the underlying code.

An API is a contract between two software systems: here is how you ask me for something, and here is what I give back.