An API — Application Programming Interface — is a way for one piece of software to talk to another. Instead of building everything from scratch, apps use APIs to request services from other apps. Every AI tool you use has an API. When you type into ChatGPT, your browser calls OpenAI's API. OpenAI's models run. The response comes back.
API = a software doorbell. You ring it with a request, something happens, you get a response back.