It is not very annoying for a visitor of your site, but a lot of crawlers are loorking for the meta key: 

<meta name="generator" content="Joomla! - Open Source Content Management" />

Unfortunately some bad guys looking for Joomla websites to take advantage of any possible security vulnerabilities.

To remove this meta key in Joomla

1. go to your current template and open the index.php
2. find the code
   

   defined('_JEXEC') or die;

   or

   defined('_JEXEC') or die('Restricted access');

3. Add
   

   JFactory::getDocument()->setGenerator('');

   right after the code or set your own Generator name:
   

   JFactory::getDocument()->setGenerator('MyGenerator');

4. Reload your site and the generator should be changed