Magento 2 use show Success and Error Message Using Interface MagentoFrameworkMessageManagerInterface.

You can set success message Using addSuccessMessage( ) and set error message using addErrorMessage( )function.

You can directly fetch Object of MagentoFrameworkMessageManagerInterface in controller file but for other files like Plugin, Observer, Model, Block or Helper; you need to instantiate an object in your __construct( ) method.

 public function __construct(
    MagentoFrameworkMessageManagerInterface $messageManager
) {
    $this->messageManager = $messageManager;
}

Now you can set messages like below:

$yourMessage = __('Your Error Message');
$this->messageManager->addErrorMessage($message);

$yourMessage = __('Your Warning Message');
$this->messageManager->addWarningMessage($message);

$yourMessage = __('Your Notice Message');
$this->messageManager->addNoticeMessage($message);