API stands for application programming interface. It is the agreed set of requests a program can make to another program and the responses it gets back. A weather API, for example, lets your code ask for a forecast and receive structured data, without knowing how the forecast is produced.
Most web APIs work over HTTP: you send a request to a URL with a method such as GET or POST, often with an authentication key, and receive data, commonly in JSON. APIs have rules you must respect, including rate limits, required fields and error codes.
They are the glue of automation and AI. A workflow calls APIs to move data between tools, and a model's tool calling typically ends up as an API call. A webhook is the reverse direction: the service calls your URL. Calls can fail or time out, so plan for error handling and retry logic.
Related: Workflow Automation, Idempotency, n8n.