OpenAPI is a specification that provides a machine-readable description of HTTP web services. It allows developers to define every possible input and output of every endpoint, making it a comprehensive tool for documenting APIs. OpenAPI can auto-generate API documentation, client libraries for various programming languages, and even server-side code, including models, attribute validation, and function scaffolding. Additionally, tools like Swagger Editor, Stoplight Elements, RapiDoc, ReDoc, and FastAPI provide interactive editing tools, renderers, and frameworks that work with OpenAPI specifications. These features make OpenAPI a valuable addition to any API project, offering benefits such as improved documentation, reduced development time, and increased collaboration among developers.