An API (Application Programming Interface) is like a waiter at a restaurant. Just like how you tell the waiter what food you want to order, a program can use an API to ask another program to do something for it. The API acts like a messenger between the two programs, taking requests from one and sending them to the other.
Imagine you're playing a game on your tablet. When you tap on the screen to make the character move, your tablet is using an API to send a message to the game's program to move the character. The API is like the waiter, taking your request and delivering it to the game program.