You can import multiple different elements into Postman via that feature.
One of those would be an API definition. Postman supports a variety of API definition types such as Protobuf, OpenAPI (formerly Swagger), WSDL, RAML, and GraphQL.
When you import these you get different options like you have seen in the UI and this API definition would be stored in the APIs section on the sidebar.
More information about the API Builder can be found here:
This may not be visible to you in the Workspace, new Workspaces only show a couple of the core elements by default and you would need to display this via the Workspace Settings.