What does this `checkSession` function do? - ionic-framework

Question

I have included the firebase and angularfire libraries to my ionic framework project.
I am following the below link for my sample project.
https://www.sitepoint.com/creating-firebase-powered-end-end-ionic-application/
I am not able to understand how the 'checkSession' function in the below 'app.js' works. Could someone please explain?
How can we call the 'auth' function which is inside 'checkSession' function?
app.js
angular.module('bucketList', ['ionic', 'firebase', 'bucketList.controllers'])
.run(function($ionicPlatform, $rootScope, $firebaseAuth, $firebase, $window, $ionicLoading) {
$ionicPlatform.ready(function() {
// Hide the accessory bar by default (remove this to show the accessory bar above the keyboard
// for form inputs)
if (window.cordova && window.cordova.plugins.Keyboard) {
cordova.plugins.Keyboard.hideKeyboardAccessoryBar(true);
}
if (window.StatusBar) {
StatusBar.styleDefault();
}
$rootScope.userEmail = null;
$rootScope.baseUrl = 'https://bucketlist-app.firebaseio.com/';
var authRef = new Firebase($rootScope.baseUrl);
$rootScope.auth = $firebaseAuth(authRef);
$rootScope.show = function(text) {
$rootScope.loading = $ionicLoading.show({
content: text ? text : 'Loading..',
animation: 'fade-in',
showBackdrop: true,
maxWidth: 200,
showDelay: 0
});
};
$rootScope.hide = function() {
$ionicLoading.hide();
};
$rootScope.notify = function(text) {
$rootScope.show(text);
$window.setTimeout(function() {
$rootScope.hide();
}, 1999);
};
$rootScope.logout = function() {
$rootScope.auth.$logout();
$rootScope.checkSession();
};
$rootScope.checkSession = function() {
var auth = new FirebaseSimpleLogin(authRef, function(error, user) {
if (error) {
// no action yet.. redirect to default route
$rootScope.userEmail = null;
$window.location.href = '#/auth/signin';
} else if (user) {
// user authenticated with Firebase
$rootScope.userEmail = user.email;
$window.location.href = ('#/bucket/list');
} else {
// user is logged out
$rootScope.userEmail = null;
$window.location.href = '#/auth/signin';
}
});
}
});
})