Mewbie finding ” the loop”

Well generally speaking, the supposed page that is generated first is the index.php file. The only code I see however is:
<?php
/* Short and sweet */
define(‘WP_USE_THEMES’, true);
require(‘./wp-blog-header.php’);
?>

Which is in my website.biz/blog

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?