The instructions for the YUN state:
"The "/arduino" directory doesn't have anything pre-configured. Anything added to the URL after the end point is passed from the webserver to the sketch on the 32U4."
The example index.html file references /arduino/temperature. Everything after the arduino.local/arduino/ part does get passed to the sketch and loaded into the variable 'command' in the line:
String command = client.readString();
You can in fact skip the html wrapper and directly try to load http://arduino.local/arduino/temperature in your browser. The 'temperature' part is passed to the sketch in the YunClient client instance. You could replace 'temperature' in the url with anything and it would be passed to the sketch where you could further process it.