what is ng-app
“The ngApp directive designates the root element of the application and is typically placed near the root element of the page – e.g. on the <body> or <html> tags.”
“AngularJS extends HTML with ng-directives.
The ng-app directive defines an AngularJS application.
The ng-model directive binds the value of HTML controls (input, select, textarea) to application data.
The ng-bind directive binds application data to the HTML view.