What is the zero index in getElementsByTagName?

Could someone tell, what happens when using this zero index?

document.getElementsByTagName('head')[0].appendChild(script);

Solution

var js = document.createElement('script');
js.src = 'myscript.js';

document.getElementsByTagName('head')[0].appendChild(js);

You get all head elements (there should be only one) and you add the script there so the result is

<html>
  <head>
    <script>
    ...

if there's no head in the document, most browsers will create the head element even if the tag is not there.