Yii2 Access Control to make all actions in controller required login by default

ExampleController.php

<?php

namespace app\controllers;

use yii\filters\AccessControl;
use yii\web\Controller;

class ExampleController extends Controller
{
    public function behaviors()
    {
        return [
            'access' => [
                'class' => AccessControl::className(),
                'rules' => [
                    [
                        'allow' => true,
                        'roles' => ['@'],
                    ],
                ],
            ],
        ];
    }

    public function actionIndex()
    {
        // Action index
    }
}

There are two symbol that m confused '?' AND '@' ... what it actually means ?

? for unknown user and @ for login user
@deshario