Spring REST Docs is a library that allows documenting RESTful web services by capturing snippets of requests and responses made through Spring MVC Test. It uses asciidoc to generate documentation from the snippets. The sample application demonstrates setting up Spring MVC Test with RestDocumentation, executing requests to capture snippets, and configuring the asciidoc Maven plugin to generate HTML documentation from the snippets. Key aspects covered include documenting request and response fields, parameters, headers and constraints.