translation please.

[villain222]

I'm having some trouble working on a site that i started a few years ago.  I'm basically relearning how it was working, or not working.  I've been able to figure out most, but I've come across something that I'm not finding online. 

What is the purpose of this $i here?

$catSQL = "SELECT * from category";
$stSQL = mysql_query($catSQL) or die ("<br><font color='red'><b>There was a problem!</b></font><br><br>" . mysql_error()); 
while ($i < mysql_num_rows($stSQL)) { 
$catname=mysql_result($stSQL,$i,"name");
$catid=mysql_result($stSQL,$i,"id");
$categoryDropdown .= "<option value=".$catid.">".$catname."</option>";
++$i;
}

[litebearer]

http://www.freewebmasterhelp.com/tutorials/phpmysql/4