Swagger UI is a collection of HTML, CSS and JavaScript objects that dynamically generate documentation from a Swagger-compliant API
Swagger UI is part of the Swagger project.
It adds form to your rest APIs by making them look good and presentable so that client code can be written easily, at the same time it also makes documentation easier by integrating it with code.
It originated out of the Open API Specification. The source can be found here.
A sample Swagger UI can be found at the Pet Store.