Open API / Swagger
An OpenAPI specification is a formal (human and computer readable) description of an API, like LAPIS. You can find the specification of this LAPIS instance here: https://lapis.cov-spectrum.org/gisaid/v2/api-docs.
Swagger UI
A Swagger UI is an interactive UI generated from the OpenAPI specification. You can find LAPIS' Swagger UI included below or behind this link: https://lapis.cov-spectrum.org/gisaid/v2/swagger-ui.html. It shows all available endpoints and allows you to try them out. Just click on the "Try it out" button. You can set the filters, fields to group by and the response format.
The Swagger UI also provides the possible responses for each endpoint, sorted by the response codes. In general, a response of 200 means that the request was successful and it returns the requested data. For json responses, example data is provided. Additionally, if you want to know more about the response and its fields, you can click on 'Schema', next to 'Example Value', and navigate through the tree of fields.