An API is like the menu in a restaurant. You pick something off the menu and you get it sent to you. You don't know what happens in the kitchen, but it gives you the options and sends you what you pick. API is the same thing. It tells you how to interact with a server/system/whatever and then sends you the info you pick off the menu (the API). You don't know what happens under the hood on their side to get the info, but you pick what you want (endpoint, parameters, etc.) and it sends it to you
The menu would be the API documentation. The waiter would be the API itself. The API is the Interface you interact with, just like the waiter is the restaurant staff you interact with. Like the waiter interacting with the kitchen staff, the API interacts with other libraries of functions that do the heavy lifting and prepare the data that the waiter/API returns to you.