Explain Root Directory with index.php

my first custome field.

Well, generally speaking the supposed page that is generated first is the index.php file, but in my case the only code I see is:
<?php
/* Short and sweet */
define(‘WP_USE_THEMES’, true);
require(‘./wp-blog-header.php’);
?>
I understand it, somewhat but I can’t get past reading and comprehending what it says. Where is the supposedly “loop” mentioned. This headache started after attempting to add custom fields in a simple post, but, it required some extra coding inside a PHP file, I think. How can I make the custom field appear?