about the example of extjs 4

[jomlim]

Ext.Loader.setPath({
'Ext.ux.desktop': 'js',
MyDesktop: ''
});
Ext.require('MyDesktop.App');
var myDesktopApp;
Ext.onReady(function () {
myDesktopApp = new MyDesktop.App();
});
does anyone explain those code ？
this is code of extjs 4 's example of desktop.html