API stands for Application Programming Interface. Which tells you absolutely nothing. So let's ignore the name and talk about what it actually does.
You're at a restaurant. You know what you want. The kitchen can make it. But you don't walk into the kitchen yourself — you tell the waiter, the waiter takes your order back to the kitchen, and comes back with your food.
That's an API. It's the waiter.
(the app)
(the waiter)
(the kitchen)
When two apps talk to each other, they do it through an API. When you connect an AI assistant — Claude, ChatGPT, Gemini, whichever you use — to Notion, or your email, or your calendar, that connection happens through an API. You're essentially giving the AI a menu of what the service can do, and it places the orders.
This matters for a practical reason: as you start using AI for more things, you'll keep hearing the word "API." It usually just means the bridge between two tools. An API key is a unique string of letters and numbers that identifies you — like a member card for the restaurant.
This website actually has its own tiny API. When you click Subscribe and type in your email, the form sends a small request across to a piece of code that lives on the same site — and that code catches your email and saves it. There's no API key in the mix this time, because keys are mostly only needed when one site needs to talk to a different site. But the same kind of conversation is happening behind the scenes. You never see it. It just works.
A note on cost, because it varies: some AI APIs have a free tier — Google's Gemini API does, for its lighter models — while most others charge by usage. Check the pricing page before you connect things, but don't let it put you off experimenting; "up to a certain amount" at hobby scale is a very different bill to running a business on it.
Facts checked July 2026.