title: ffmpeg map filter option
body: <p>What does below ffmpeg option do?</p>
  
  <pre><code> -map [v] -map [a]
  </code></pre>
  
  <p>and</p>
  
  <pre><code> -map 0
  </code></pre>
  
  <p>I have read <a href="https://ffmpeg.org/ffmpeg.html#Advanced-Video-options" rel="nofollow noreferrer">this</a> but still not able to understand exactly what it does and when to use them?</p>