An API or Application Programming Interface is like a waiter at a restaurant. When you go to a restaurant, you look at the menu and order what you want to eat. The waiter then takes that order to the kitchen and brings your food back to you.
Similarly, when you use an API you are like the customer in a restaurant. You know what you want to do with the software or online service, but you don't know how to do it yourself. The API is the waiter who takes your order (your input or request) and takes it to the kitchen (the back-end server or database).
The API then comes back to you with the output (the data, result or action you requested). It's like you telling the waiter, "I want a burger" and the waiter coming back shortly with a hot, juicy burger for you.
APIs are a way for programs and applications to communicate and exchange information with each other. With an API, you can request data or perform actions on a server or database without needing to know how the backend processes work.
In summary, an API is like a waiter that takes your orders or requests, goes to the kitchen or the backend system, and brings back what you asked for.