AngularJS uses Routes. #/secondHTML is a route, which is usually mapped to a template partial, which contains the HTML needed to render the view.
Usually inside app.js is where the code that configures your routes will be.
The code probably looks like:
angular.module('isweb', [
'isweb.controllers',
'isweb.filters',
'isweb.services',
'isweb.directives',
]).
config(['$routeProvider','$locationProvider',function ($routeProvider, $locationProvider) {
$routeProvider.
when('/secondHTML', {
templateUrl: 'partials/page2',
controller: 'page2Ctrl'
}).
otherwise({
redirectTo: '/firstHTML'
});
}]);
Notice the config method, which takes the routeProvider, as well as the mappings between the URL and the actual URL for the partial template.
So when you go to: #/secontHTML AngularJS will actually request partials/page2 from the server, and render that. However, the address bar will look something like myapp.com/app#/secondHTML
I HIGHLY recommend going through the AngularJS tutorial.