An API is a way for two separate programs to talk to each other and exchange information automatically, without a person copying anything by hand. It is a set of agreed requests one program can make and answers the other gives back.
A familiar picture is a waiter in a restaurant: you do not walk into the kitchen yourself — you tell the waiter what you want, and the waiter brings it back from the kitchen. An API is that waiter between programs: your website asks for something (say, today’s exchange rate or a delivery price), and another service returns the answer, following the agreed rules.
APIs are how websites connect to outside services — payment systems, maps, delivery companies — and they are also how an AI agent can plug into and act on other tools.