Customer Authentication in Magento 2 is required when we have to check the customer is a guest user or a login user.

There are many occasions when we have to check the customer is login or not, and based on the login customer, only we have to show a specific page; otherwise, redirect it to the login page.

To verify whether it’s a login customer or not,
use MagentoCustomerModelSession Class and add the Customer Session class to the __construct() method of your Controller.

<?php

namespace MagemonkeysFavoriteListControllerIndex;

use MagentoFrameworkAppAction;
use MagentoCustomerModelSession;
use MagentoFrameworkViewResultPage;
use MagentoFrameworkAppActionContext;
use MagentoFrameworkControllerResultFactory;

class Index extends Action
{
    /**
     * @var Session
     */
    protected $customerSession;

    public function __construct(
        Context $context,
        Session $customerSession
    ) {
        $this->customerSession = $customerSession;
        parent::__construct($context);
    }

    /**
     * Prepare wishlist for share
     *
     * @return ResultInterface
     */
    public function execute()
    {
        if (!$this->customerSession->authenticate()) {
            /** @var Page $resultPage */
            $result = $this->resultFactory->create(ResultFactory::TYPE_REDIRECT);
            $result->setPath('customer/account/login');
            return $result;
        }

        // do your logic here
    }
}

Using this approach, we can check the customer is a login customer or not.