Moodle can use an additional level of security when accepting data from web forms. If this is enabled, then the browser's HTTP_REFERER variable is checked against the current form address.  In a very few cases this can cause problems if the user is using a firewall (eg Zonealarm) configured to strip HTTP_REFERER from their web traffic.  Symptoms are getting 'stuck' on a form. If your users are having problems with the login page (for example) you might want to disable this setting, although it might leave your site more open to brute-force password attacks.  If in doubt, leave this set to 'Yes'.