The OpenAPI Specification (OAS) is a specification for describing a REST web service using a JSON or YAML file (readable by both humans and machines) that specifies the URLs that can be used, the HTTP operations allowed, the parameters to be passed, the response formats... This is essential documentation for the customer wishing to access the service.
OpenAPI was born as part of the Swagger suite but since 2016 it is a separate open-source specification, managed by a project of the Linux Foundation (OpenAPI Initiative). The Swagger suite provides many tools to edit a service description (Swagger Editor), transform the JSON or YAML file into interactive documentation (Swagger UI), or generate the server code skeleton or an access library for the client (Swagger Codegen). OAS is a format recognized by tools other than those of the Swagger suite, both open-course or commercial.
Read this page that walks you through an OAS document.