title: What does "using System" mean in C#?
body: <p>What does</p>
  <pre><code>using System;
  
  namespace HelloWorld
  {
    class Program
    {
      static void Main(string[] args)
      {
        Console.WriteLine(&quot;Hello World!&quot;);
      }
    }
  }
  </code></pre>
  <p>[&quot;using System&quot;] mean? Why can't you start your code without these lines?</p>