The white box says this:
Access disallowed. You are visiting from "" while authorized users should come only from "http://natas5.natas.labs.overthewire.org/" Refresh page
Clicking on "Refresh page" redirects us to http://natas4.natas.labs.overthewire.org/index.php (it really refreshes the page). This page says:
Clicking again on Refresh page refreshes the page. And the page now says:
Access disallowed. You are visiting from "http://natas4.natas.labs.overthewire.org/index.php" while authorized users should come only from "http://natas5.natas.labs.overthewire.org/"
It seems that the page uses the referer to know which page your are coming from, and will only give you the password if you come from http://natas5.natas.labs.overthewire.org/.
<div id="viewsource"><a href="http://natas4.natas.labs.overthewire.org">Refresh page</a></div>
Now when I click on the Refresh page button I added, I am redirected to the natas4 page (yay!) and here is what it says:
Access granted. The password for natas5 is iX6IOfmpN7AYOQGPwtn3fXpbaJVJcHfq