Buttondown Documentation
OpenAPI is a JSON-based specification for building APIs. It sounds an awful lot like OpenAI (the very popular company behind GPT-3 and ChatGPT and all that stuff), but it is completely unrelated.
Many publicly-facing APIs offer a public "OpenAPI spec" that describes the API's endpoints, request and response formats, and other details. This spec can be used to generate client libraries, documentation, and other tools.
You can find Buttondown's here.