The OpenAPI Specification (OAS) defines a standard, programming language-agnostic interface description for HTTP APIs, which allows both humans and computers to discover and understand the capabilities of a service without requiring access to source code, additional documentation, or inspection of network traffic. When properly defined via OpenAPI, a consumer can understand and interact with the remote service with a minimal amount of implementation logic.
Use cases for machine-readable API definition documents include, but are not limited to: interactive documentation; code generation for documentation, clients, and servers; and automation of test cases. OpenAPI documents describe an APIs services and are represented in either YAML or JSON formats. These documents may either be produced and served statically or be generated dynamically from an application.
There are implementations available for many languages like C#, including low-level tooling, editors, user interfaces, code generators, etc. Here you can find a list of known tooling for the different languages: OpenAPI-Specification/IMPLEMENTATIONS.md.