To use it, just paste your original source code into the *Your source code* box and click on the *GO* button. The source code will be sent to the Closure Compiler (by Google) online service to get compiled. The result will be shown in the *Compiled code* box. If your source code contains errors or warnings, the error or warning messages will be shown instead of the compiled code, and it will stop there. If your source code is good, the compiled code will be encoded and the result will be shown in the last *Encoded code* box.
If you know the warnings returned from the Closure Compiler is trivial and you can live with it -- not recommended, you can let the tool ignore the warnings by checking *Ignore Warnings*.
This tool is not supported in the old browsers, e.g. Firefox 3.6. If it does not work on your browser, please try upgrade your browser. It does not work on IE 9. IE 10 starts to support XMLHttpRequest so most likely it could work on IE 10.