Mark - define hard codes the site and home url to allow access in these scenarios, the DB options are overridden. This allows you to login and change the options. You then remove the define to return to using the DB options.
Using update_options would not work as wp-config is the loaded before the options API, though at the moment the goal is to get back up and running, quickly so this route is simpler then explaining PHPMyAdmin
GSalgaller - the wp-config.php file will be in the root of your site (so using your FTP client or your hosts admin panel to access them) normally, it may also be one level higher. So if you have a public_html/wp folder it could be in either wp or public_html.
Once you locate it the original 2 lines I suggested are the correct way forward.