$content = ob_get_contents();这句代码怎么理解

<?php

setcookie('test', time());

ob_start();

print_r($_COOKIE); 

$content = ob_get_contents();

$content = str_replace(" ", '&nbsp;', $content);

ob_clean();

header("content-type:text/html; charset=utf-8");

echo '当前的Cookie为：<br>';

echo nl2br($content);