title: php __construct() - please explain what the argument means
body: <p>I searched for an answer but I couldn't find. I know the concept of classes, objects, constructors etc.</p>
  
  <p>What I don't understand is, this kind of syntax in PHP.</p>
  
  <pre><code>public function __construct(\Magento\Config\Model\ResourceModel\Config $resourceConfig) {
      $this-&gt;resourceConfig = $resourceConfig;
  }
  </code></pre>
  
  <p>Specifically I don't understand the argument of constructor. Where it comes from? From another class? Which file should I look for $resourceConfig?</p>
  
  <p>Could someone please explain? Thank you in advance!  </p>