An API makes it possible for one software to interact with other software. So it’s basically a code that allows communication between two separate apps. So, for instance, when you book a hotel from your mobile phone through an app, the API takes your booking request to the system and returns it with a response to your phone. So in this case, a booking confirmation.
Another good analogy is to think about API as a waiter. When you’re in a restaurant and ready to order your food off the menu, you need the waiter to send your request to the kitchen and come back with your order.