I'm having problems figuring out this javascript:

<HTML>
<HEAD>
<TITLE>Null</TITLE>
<SCRIPT>

var string = "8ac249fbd363fx52j1";
var enigma = "";
var whatisthis = "var myxor = prompt('Password:','');

for (y=1; y<5; y++)
{
enigma += (string.indexOf(y)+1);
}
enigma += 5;
if (myxor==enigma)
{
enigma = enigma + '.php';location.href=enigma;
}
else{location.href='hahaha.php';
}";

eval(whatisthis);

</SCRIPT>
</HEAD>
<BODY></BODY>
</HTML>

I kind of separated it a little bit for it to be somewhat understandable. For those who might recognize it, this is some source code for the first level of the Mod-X wargame. I'm a real newbie at this and can't get some of it....mainly I don't understand what += means. Can anyone help?

[Edit: This is me assuming this is javascript. I mean, I see the script tag, but I don't know if it's java. Just a naive assumption. Correct me if I'm wrong]