Swagger
In plain terms
A toolset for designing, documenting, and testing REST APIs.
Going deeper
Swagger provides UI and tooling around the OpenAPI spec to visualize, try, and generate code for APIs. It made interactive API docs mainstream.
Under the hood
Swagger UI renders the spec as explorable docs; the spec itself became OpenAPI.
Examples
Interactive API docs, Swagger UI, API design, try-it-out consoles.
Think of it like...
Like an interactive instruction manual you can press buttons on to test the product.