UrlRewriting means the way parameters are thrown into the WebBuilder2.

Normally you have:
www.somehost.org/index.php?foo=bar (via GET)

in WebBuilder2 you have the opportunity to transfer these parameters this way:
www.somehost.org/foo/bar

The WebBuilder2 builds then these variables in the parameter_array as a key=>value pair.

You can define if you wish to have some special variables on fixed positions and you would need only to give the value for this variable:

Example:
www.somehost.org/somepage

If you have defined:
$GLOBALS['core']['mod_rewrite'][0] = 'pid'; 
in the core.ini (see ConfigurationFiles)
then somepage will be the value of the page_id.