title: What are this, it, activity in kotlin android exactly?
body: <p>I am new to android and following the official Android guide.
  Sometimes I can't understand some keywords in the example codes,
  like <code>it</code>, <code>this</code>, and <code>activity</code>.</p>
  
  <p>Suppose I have to call a dialog like this:</p>
  
  <pre><code>SampleDialog().show(this@MainActivity.supportFragmentManager, null)
  </code></pre>
  
  <p>What does <code>this@Activity</code> mean?</p>