Register globals is a feature that will register all variables passed to a script as global variables. Because of the security implications this will bring along, it was turned off by default since php 4.2.0. Most hosting companies still have this enabled though. (read more on php.net)

It's advised to turn register globals off, to be more secure against hacking attempts and poorly built components/modules and plugins.

To turn this off, open your php.ini (if you have access to it, otherwise convince your host to do it). Search for the line:

register_globals = On

and change it to:

register_globals = Off

Then restart apache.

If for some reason, your hosting provider doesn't want to disable it, there's a way to overwrite the setting by using a .htaccess file. Open your current .htaccess file in the root of Joomla and add to it:

php_flag register_globals off

That should turn it off. If this doesn't work, contact your hosting provider and ask them to configure their server so you can overwrite this setting.