API
also: apis, application programming interface
A set of rules that lets one piece of software ask another for information or actions. It's how apps talk to each other.
An Application Programming Interface: a defined set of operations one program offers to others, including what to send and what comes back. Web APIs let apps use services like payments, maps, email or AI over the internet.
A restaurant menu works like an API. It lists what you can order and how to ask for it. You don't walk into the kitchen; you order from the menu and your dish arrives.
Apps built with agents lean on outside APIs for payments, maps, email and AI. Knowing what an API is explains why a feature depends on another company's service, and why it can break without anyone touching your code.
I'll use Stripe's API to create the payment. Your app sends the amount, and the card details go straight to Stripe, never through your server.
Use the weather service's API to show tomorrow's forecast on the event page. Handle the case where the API is down.
Thinking an API is a website or app you open. It's the agreed way for programs to ask each other for things, not something people usually look at.