Firstly on your last post. The first line uses a server variable. A server variable is information about the server environment. For example, the server name, the IP of the visiting user, your browser type, etc. In this case _SERVER["PHP_SELF] is the name of the page that has been called.

Here is PHPINFO - A built in command in PHP that will display all the server variables and environment information. http://www.digital-end.com/phpinfo.php
You can do this on your own server using the phpinfo() function.

Now as for isset, it does exactly what it says on the tin! It checks if a variable exists. If it does then the contents of the if statement are executed. If not, then...not!

By now you can probably guess what $_SERVER['QUERY_STRING'] is? It's the query strings (the variables passed via an URL. Eg: mypage.php?page=contact

$editFormAction .= "?" . htmlentities($_SERVER['QUERY_STRING']);

This is actually quite simple... this creates (or changes) a variable called editFormAction to be a question mark and whatever query string has been passed. htmlentities simply converts all characters to HTML entities, so that spaces become %20 and so on.
The idea here, I presume, is that a form is sent to the relevant place. Basically, this script can display a form and then send it to any number of places depending on what query string you have passed. So in order to go to contact_success.php just put that as the query string. It appears that the querystring should be $theValue, but this script IS very confusing, badly written and has no comments at all! There should at least be a short description of what the script does as well as comments to which variables do what, etc.

I can't determine what insertgoto does?