@RequestMapping(/)

This is a Spring mapping annotation that specifies the root URL mapping for a controller class or a method within a controller class.

For example, if you have a controller class with the mapping annotation `@RestController` and `@RequestMapping("/")`, all requests to the root URL of the application will be handled by methods in this controller class.

If you have a method within this controller class with the mapping annotation `@RequestMapping("/hello")`, requests to the URL `http://localhost:8080/hello` will be handled by this method.

The `/` in the `@RequestMapping("/")` denotes the root URL of the application. You can replace this with any other URL path to map requests to a different URL path.