What is the purpose of square bracket usage in Angular?

I would like to understand the difference between the declaration of MyOtherService and MyOtherComplexService. Especially what is the purpose of square bracket part? When to use them and when not?

var myapp = angular.module('myapp', []);

myapp.factory('MyService', function($rootScope, $timeout) {
    return {
        foo: function() {
            return "MyService";
        }
    }
});

myapp.factory('MyOtherService', function($rootScope, $timeout, MyService) {
    return {
        foo: function() {
            return "MyOtherService";
        }
    }
});

myapp.factory('MyOtherComplexService', ['$rootScope', '$timeout', 'MyService', function($rootScope, $timeout, MyService) {
    return {
        foo: function() {
            return "MyOtherComplexService";
        }
    } 
}]);

myapp.controller('MyController', function($scope, MyOtherService, MyOtherComplexService) {

    $scope.x = MyOtherService.foo();
    $scope.y = MyOtherComplexService.foo(); 
});