What is an API?

A defined way for one system to ask another for something or tell it to do something. It matters to an operator for one reason: whether the software you already run has a usable one decides how cheaply it can be connected to anything else. When a vendor says integration is not possible, they usually mean the API is missing, restricted to a higher tier, or read-only. That is worth establishing before you plan any work around a system, not after.