The “OB” stands for “output buffer”, and essentially anything that comes after the “ob_start” and before the “ob_get_clean” is what’s returned in the function.
<?php
function theHTML( $attributes ) {
ob_start(); ?>
<h1> Fun with HTML!</h1>
<p> This is PHP <?php echo esc_html($attributes['theThing']) ?> So great!</p>
<?php return ob_get_clean();
}