An API, short for Application Programming Interface, is a set of rules that lets one software system ask another for data or an action. If a website is a storefront built for people, an API is the loading dock built for other software, a structured entrance meant for machines instead of customers.
APIs are what make modern software work together. Your accounting tool can pull sales from your store, your shipping app can read orders from your ERP, and an AI feature can call a language model, all of it automatic, because each system exposes an API the others can call.
For a business, APIs decide whether your tools can share data or trap it in silos. When you are sizing up software, "does it have an API?" is really asking "will this play nice with the rest of my stack, now and later?" Clean, well-documented APIs are a sign of software built to connect, not to lock you in.